diff --git a/misc/pprof b/misc/pprof
index 777a45cb7c..2fe56503c9 100755
--- a/misc/pprof
+++ b/misc/pprof
@@ -1,7 +1,7 @@
 #! /usr/bin/env perl
 
 # This is a copy of http://google-perftools.googlecode.com/svn/trunk/src/pprof
-# with local modifications to handle generation of SVG images and 
+# with local modifications to handle generation of SVG images and
 # the Go-style pprof paths.  These modifications will probably filter
 # back into the official source before long.
 # It's convenient to have a copy here because we need just the one
@@ -9,11 +9,11 @@
 
 # Copyright (c) 1998-2007, Google Inc.
 # All rights reserved.
-# 
+#
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
-# 
+#
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
@@ -23,7 +23,7 @@
 #     * Neither the name of Google Inc. nor the names of its
 # contributors may be used to endorse or promote products derived from
 # this software without specific prior written permission.
-# 
+#
 # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 # "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 # LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
@@ -1234,6 +1234,13 @@ sub Disassemble {
   my $cmd = sprintf("$objdump -C -d -l --no-show-raw-insn " .
                     "--start-address=0x$start_addr " .
                     "--stop-address=0x$end_addr $prog");
+
+  if (system("$objdump --help >/dev/null 2>&1") != 0) {
+    # objdump must not exist.  Fall back to go tool objdump.
+    $objdump = "go tool objdump";
+    $cmd = "$objdump $prog 0x$start_addr 0x$end_addr";
+  }
+
   open(OBJDUMP, "$cmd |") || error("$objdump: $!\n");
   my @result = ();
   my $filename = "";
@@ -1305,10 +1312,10 @@ sub PrintListing {
   my $cumulative = shift;
   my $list_opts = shift;
   my $html = shift;
-  
+
   my $output = \*STDOUT;
   my $fname = "";
-  
+
 
   if ($html) {
     # Arrange to write the output to a temporary file
@@ -1323,7 +1330,7 @@ sub PrintListing {
     printf $output ("<div class=\"legend\">%s<br>Total: %s %s</div>\n",
                     $main::prog, Unparse($total), Units());
   }
- 
+
   my $listed = 0;
   foreach my $lib (@{$libs}) {
     my $symbol_table = GetProcedureBoundaries($lib->[0], $list_opts);
@@ -2221,7 +2228,7 @@ function handleMouseWheel(evt) {
 		z = 0.1;
 	if(z > 10.0)
 		z = 10.0;
-		
+
 	var g = svgDoc.getElementById("viewport");
 
 	var p = getEventPoint(evt);
@@ -4391,6 +4398,12 @@ sub MapToSymbols {
     $cmd = "$addr2line --demangle -f -C -e $image";
   }
 
+  if (system("$addr2line --help >/dev/null 2>&1") != 0) {
+    # addr2line must not exist.  Fall back to go tool addr2line.
+    $addr2line = "go tool addr2line";
+    $cmd = "$addr2line $image";
+  }
+
   # If "addr2line" isn't installed on the system at all, just use
   # nm to get what info we can (function names, but not line numbers).
   if (system("$addr2line --help >/dev/null 2>&1") != 0) {
@@ -4434,7 +4447,7 @@ sub MapToSymbols {
   if ($debug) {
     print("----\n");
     system("cat $main::tmpfile_sym");
-    print("----\n");
+    print("---- $cmd\n");
     system("$cmd <$main::tmpfile_sym");
     print("----\n");
   }
@@ -4544,7 +4557,7 @@ sub ShortFunctionName {
 # Trim overly long symbols found in disassembler output
 sub CleanDisassembly {
   my $d = shift;
-  while ($d =~ s/(?<!\.)\([^()%]*\)(\s*const)?//g) { } # Argument types, not (%rax)
+  while ($d =~ s/(?<!\.)\([^()%A-Z]*\)(\s*const)?//g) { } # Argument types, not (%rax)
   while ($d =~ s/(\w+)<[^<>]*>/$1/g)  { }       # Remove template arguments
   return $d;
 }
@@ -4625,7 +4638,7 @@ sub ConfigureTool {
     my $dirname = $`;    # this is everything up to and including the last slash
     if (-x "$dirname$tool") {
       $path = "$dirname$tool";
-    } else { 
+    } else {
       $path = $tool;
     }
   }
diff --git a/src/cmd/addr2line/main.c b/src/cmd/addr2line/main.c
new file mode 100644
index 0000000000..6b2fe5dfe1
--- /dev/null
+++ b/src/cmd/addr2line/main.c
@@ -0,0 +1,68 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+ * addr2line simulation - only enough to make pprof work on Macs
+ */
+
+#include <u.h>
+#include <libc.h>
+#include <bio.h>
+#include <mach.h>
+
+void
+usage(void)
+{
+	fprint(2, "usage: addr2line binary\n");
+	fprint(2, "reads addresses from standard input and writes two lines for each:\n");
+	fprint(2, "\tfunction name\n");
+	fprint(2, "\tfile:line\n");
+	exits("usage");
+}
+
+void
+main(int argc, char **argv)
+{
+	int fd;
+	char *p;
+	uvlong pc;
+	Symbol s;
+	Fhdr fhdr;
+	Biobuf bin, bout;
+	char file[1024];
+
+	ARGBEGIN{
+	default:
+		usage();
+	}ARGEND
+
+	if(argc != 1)
+		usage();
+
+	fd = open(argv[0], OREAD);
+	if(fd < 0)
+		sysfatal("open %s: %r", argv[0]);
+	if(crackhdr(fd, &fhdr) <= 0)
+		sysfatal("crackhdr: %r");
+	machbytype(fhdr.type);
+	if(syminit(fd, &fhdr) <= 0)
+		sysfatal("syminit: %r");
+
+	Binit(&bin, 0, OREAD);
+	Binit(&bout, 1, OWRITE);
+	for(;;) {
+		p = Brdline(&bin, '\n');
+		if(p == nil)
+			break;
+		p[Blinelen(&bin)-1] = '\0';
+		pc = strtoull(p, 0, 16);
+		if(!findsym(pc, CTEXT, &s))
+			s.name = "??";
+		if(!fileline(file, sizeof file, pc))
+			strcpy(file, "??:0");
+		Bprint(&bout, "%s\n%s\n", s.name, file);
+	}
+	Bflush(&bout);
+	exits(0);
+}
diff --git a/src/cmd/dist/build.c b/src/cmd/dist/build.c
index 5664c1890a..66b5c1f183 100644
--- a/src/cmd/dist/build.c
+++ b/src/cmd/dist/build.c
@@ -59,7 +59,7 @@ int
 find(char *p, char **l, int n)
 {
 	int i;
-	
+
 	for(i=0; i<n; i++)
 		if(streq(p, l[i]))
 			return i;
@@ -73,7 +73,7 @@ init(void)
 	char *p;
 	int i;
 	Buf b;
-	
+
 	binit(&b);
 
 	xgetenv(&b, "GOROOT");
@@ -126,7 +126,7 @@ init(void)
 	xsetenv("GOROOT", goroot);
 	xsetenv("GOARCH", goarch);
 	xsetenv("GOOS", goos);
-	
+
 	// Make the environment more predictable.
 	xsetenv("LANG", "C");
 	xsetenv("LANGUAGE", "en_US.UTF8");
@@ -170,13 +170,13 @@ findgoversion(void)
 	int i, nrev;
 	Buf b, path, bmore, branch;
 	Vec tags;
-	
+
 	binit(&b);
 	binit(&path);
 	binit(&bmore);
 	binit(&branch);
 	vinit(&tags);
-	
+
 	// The $GOROOT/VERSION file takes priority, for distributions
 	// without the Mercurial repo.
 	bpathf(&path, "%s/VERSION", goroot);
@@ -232,14 +232,14 @@ findgoversion(void)
 		bprintf(&b, "branch.%s", bstr(&branch));
 		tag = btake(&b);
 	}
-	
+
 	if(rev[0]) {
 		// Tag is before the revision we're building.
 		// Add extra information.
 		run(&bmore, goroot, CheckExit, "hg", "log", "--template", " +{node|short}", "-r", rev, nil);
 		chomp(&bmore);
 	}
-	
+
 	bprintf(&b, "%s", tag);
 	if(bmore.len > 0)
 		bwriteb(&b, &bmore);
@@ -249,14 +249,14 @@ findgoversion(void)
 
 done:
 	p = btake(&b);
-	
-	
+
+
 	bfree(&b);
 	bfree(&path);
 	bfree(&bmore);
 	bfree(&branch);
 	vfree(&tags);
-	
+
 	return p;
 }
 
@@ -325,7 +325,7 @@ setup(void)
 			xremoveall(p);
 		xmkdirall(p);
 	}
-	
+
 	// Create object directory.
 	// We keep it in pkg/ so that all the generated binaries
 	// are in one tree.  If pkg/obj/libgc.a exists, it is a dreg from
@@ -393,7 +393,7 @@ static char *proto_gccargs[] = {
 static Vec gccargs;
 
 // deptab lists changes to the default dependencies for a given prefix.
-// deps ending in /* read the whole directory; deps beginning with - 
+// deps ending in /* read the whole directory; deps beginning with -
 // exclude files with that prefix.
 static struct {
 	char *prefix;  // prefix of target
@@ -559,7 +559,7 @@ install(char *dir)
 	vinit(&clean);
 	vinit(&lib);
 	vinit(&extra);
-	
+
 	// path = full path to dir.
 	bpathf(&path, "%s/src/%s", goroot, dir);
 	name = lastelem(dir);
@@ -599,7 +599,7 @@ install(char *dir)
 	exe = "";
 	if(streq(gohostos, "windows"))
 		exe = ".exe";
-	
+
 	// Start final link command line.
 	// Note: code below knows that link.p[targ] is the target.
 	if(islib) {
@@ -696,13 +696,13 @@ install(char *dir)
 					}
 					files.len = n;
 					continue;
-				}				
+				}
 				vadd(&files, p);
 			}
 		}
 	}
 	vuniq(&files);
-	
+
 	// Convert to absolute paths.
 	for(i=0; i<files.len; i++) {
 		if(!isabs(files.p[i])) {
@@ -740,11 +740,11 @@ install(char *dir)
 		files.p[n++] = files.p[i];
 	}
 	files.len = n;
-	
+
 	for(i=0; i<lib.len && !stale; i++)
 		if(mtime(lib.p[i]) > ttarg)
 			stale = 1;
-		
+
 	if(!stale)
 		goto out;
 
@@ -792,9 +792,9 @@ install(char *dir)
 		copy(bpathf(&b, "%s/zasm_GOOS_GOARCH.h", workdir),
 			bpathf(&b1, "%s/zasm_%s_%s.h", bstr(&path), goos, goarch), 0);
 	}
-	
+
 	// Generate .c files from .goc files.
-	if(streq(dir, "pkg/runtime")) {		
+	if(streq(dir, "pkg/runtime")) {
 		for(i=0; i<files.len; i++) {
 			p = files.p[i];
 			if(!hassuffix(p, ".goc"))
@@ -808,7 +808,7 @@ install(char *dir)
 		}
 		vuniq(&files);
 	}
-	
+
 	if((!streq(goos, gohostos) || !streq(goarch, gohostarch)) && isgo) {
 		// We've generated the right files; the go command can do the build.
 		if(vflag > 1)
@@ -833,13 +833,13 @@ install(char *dir)
 				vadd(&compile, "-m32");
 			if(streq(dir, "lib9"))
 				vadd(&compile, "-DPLAN9PORT");
-	
+
 			vadd(&compile, "-I");
 			vadd(&compile, bpathf(&b, "%s/include", goroot));
-			
+
 			vadd(&compile, "-I");
 			vadd(&compile, bstr(&path));
-	
+
 			// lib9/goos.c gets the default constants hard-coded.
 			if(streq(name, "goos.c")) {
 				vadd(&compile, bprintf(&b, "-DGOOS=\"%s\"", goos));
@@ -849,7 +849,7 @@ install(char *dir)
 				vadd(&compile, bprintf(&b, "-DGOROOT=\"%s\"", bstr(&b1)));
 				vadd(&compile, bprintf(&b, "-DGOVERSION=\"%s\"", goversion));
 			}
-	
+
 			// gc/lex.c records the GOEXPERIMENT setting used during the build.
 			if(streq(name, "lex.c")) {
 				xgetenv(&b, "GOEXPERIMENT");
@@ -867,7 +867,7 @@ install(char *dir)
 			vadd(&compile, workdir);
 			vadd(&compile, bprintf(&b, "-DGOOS_%s", goos));
 			vadd(&compile, bprintf(&b, "-DGOARCH_%s", goarch));
-		}	
+		}
 
 		bpathf(&b, "%s/%s", workdir, lastelem(files.p[i]));
 		doclean = 1;
@@ -893,7 +893,7 @@ install(char *dir)
 			vadd(&clean, bstr(&b));
 	}
 	bgwait();
-	
+
 	if(isgo) {
 		// The last loop was compiling individual files.
 		// Hand the Go files to the compiler en masse.
@@ -905,16 +905,16 @@ install(char *dir)
 		vadd(&compile, bstr(&b));
 		vadd(&clean, bstr(&b));
 		vadd(&link, bstr(&b));
-		
+
 		vadd(&compile, "-p");
 		if(hasprefix(dir, "pkg/"))
 			vadd(&compile, dir+4);
 		else
 			vadd(&compile, "main");
-		
+
 		if(streq(dir, "pkg/runtime"))
 			vadd(&compile, "-+");
-		
+
 		vcopy(&compile, go.p, go.len);
 
 		runv(nil, bstr(&path), CheckExit, &compile);
@@ -980,7 +980,7 @@ shouldbuild(char *file, char *dir)
 	int i, j, ret;
 	Buf b;
 	Vec lines, fields;
-	
+
 	// Check file name for GOOS or GOARCH.
 	name = lastelem(file);
 	for(i=0; i<nelem(okgoos); i++)
@@ -989,11 +989,11 @@ shouldbuild(char *file, char *dir)
 	for(i=0; i<nelem(okgoarch); i++)
 		if(contains(name, okgoarch[i]) && !streq(okgoarch[i], goarch))
 			return 0;
-	
+
 	// Omit test files.
 	if(contains(name, "_test"))
 		return 0;
-	
+
 	// cmd/go/doc.go has a giant /* */ comment before
 	// it gets to the important detail that it is not part of
 	// package main.  We don't parse those comments,
@@ -1046,7 +1046,7 @@ out:
 	bfree(&b);
 	vfree(&lines);
 	vfree(&fields);
-	
+
 	return ret;
 }
 
@@ -1055,7 +1055,7 @@ static void
 copy(char *dst, char *src, int exec)
 {
 	Buf b;
-	
+
 	if(vflag > 1)
 		xprintf("cp %s %s\n", src, dst);
 
@@ -1070,11 +1070,13 @@ static char *buildorder[] = {
 	"lib9",
 	"libbio",
 	"libmach",
-	
+
 	"misc/pprof",
 
+	"cmd/addr2line",
 	"cmd/cov",
 	"cmd/nm",
+	"cmd/objdump",
 	"cmd/pack",
 	"cmd/prof",
 
@@ -1122,12 +1124,12 @@ static char *buildorder[] = {
 	"pkg/go/scanner",
 	"pkg/go/ast",
 	"pkg/go/parser",
-	"pkg/go/build",
 	"pkg/os/exec",
 	"pkg/net/url",
 	"pkg/text/template/parse",
 	"pkg/text/template",
 	"pkg/go/doc",
+	"pkg/go/build",
 	"cmd/go",
 };
 
@@ -1147,11 +1149,13 @@ static char *cleantab[] = {
 	"cmd/8c",
 	"cmd/8g",
 	"cmd/8l",
+	"cmd/addr2line",
 	"cmd/cc",
 	"cmd/cov",
 	"cmd/gc",
 	"cmd/go",
 	"cmd/nm",
+	"cmd/objdump",
 	"cmd/pack",
 	"cmd/prof",
 	"lib9",
@@ -1204,11 +1208,11 @@ clean(void)
 	int i, j, k;
 	Buf b, path;
 	Vec dir;
-	
+
 	binit(&b);
 	binit(&path);
 	vinit(&dir);
-	
+
 	for(i=0; i<nelem(cleantab); i++) {
 		bpathf(&path, "%s/src/%s", goroot, cleantab[i]);
 		xreaddir(&dir, bstr(&path));
@@ -1227,7 +1231,7 @@ clean(void)
 	if(rebuildall) {
 		// Remove object tree.
 		xremoveall(bpathf(&b, "%s/pkg/obj/%s_%s", goroot, gohostos, gohostarch));
-	
+
 		// Remove installed packages and tools.
 		xremoveall(bpathf(&b, "%s/pkg/%s_%s", goroot, gohostos, gohostarch));
 		xremoveall(bpathf(&b, "%s/pkg/%s_%s", goroot, goos, goarch));
@@ -1294,7 +1298,7 @@ cmdenv(int argc, char **argv)
 
 	if(argc > 0)
 		usage();
-	
+
 	xprintf(format, "GOROOT", goroot);
 	xprintf(format, "GOBIN", gobin);
 	xprintf(format, "GOARCH", goarch);
@@ -1346,7 +1350,7 @@ cmdbootstrap(int argc, char **argv)
 		clean();
 	goversion = findgoversion();
 	setup();
-	
+
 	// For the main bootstrap, building for host os/arch.
 	oldgoos = goos;
 	oldgoarch = goarch;
@@ -1356,7 +1360,7 @@ cmdbootstrap(int argc, char **argv)
 	gochar = gohostchar;
 	xsetenv("GOARCH", goarch);
 	xsetenv("GOOS", goos);
-	
+
 	for(i=0; i<nelem(buildorder); i++) {
 		install(bprintf(&b, buildorder[i], gohostchar));
 		if(!streq(oldgochar, gohostchar) && xstrstr(buildorder[i], "%s"))
@@ -1381,7 +1385,7 @@ defaulttarg(void)
 {
 	char *p;
 	Buf pwd, src, real_src;
-	
+
 	binit(&pwd);
 	binit(&src);
 	binit(&real_src);
@@ -1404,7 +1408,7 @@ defaulttarg(void)
 	bfree(&pwd);
 	bfree(&src);
 	bfree(&real_src);
-	
+
 	return p;
 }
 
@@ -1421,7 +1425,7 @@ cmdinstall(int argc, char **argv)
 	default:
 		usage();
 	}ARGEND
-	
+
 	if(argc == 0)
 		install(defaulttarg());
 
@@ -1469,7 +1473,7 @@ cmdbanner(int argc, char **argv)
 	binit(&b);
 	binit(&b1);
 	binit(&search);
-	
+
 	xprintf("\n");
 	xprintf("---\n");
 	xprintf("Installed Go for %s/%s in %s\n", goos, goarch, goroot);
@@ -1490,7 +1494,7 @@ cmdbanner(int argc, char **argv)
 			"On OS X the debuggers must be installed setgrp procmod.\n"
 			"Read and run ./sudo.bash to install the debuggers.\n");
 	}
-	
+
 	if(!streq(goroot_final, goroot)) {
 		xprintf("\n"
 			"The binaries expect %s to be copied or moved to %s\n",
diff --git a/src/cmd/objdump/main.c b/src/cmd/objdump/main.c
new file mode 100644
index 0000000000..b684be7fbe
--- /dev/null
+++ b/src/cmd/objdump/main.c
@@ -0,0 +1,68 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+ * objdump simulation - only enough to make pprof work on Macs
+ */
+
+#include <u.h>
+#include <libc.h>
+#include <bio.h>
+#include <mach.h>
+
+void
+usage(void)
+{
+	fprint(2, "usage: objdump binary start stop\n");
+	fprint(2, "Disassembles binary from PC start up to stop.\n");
+	exits("usage");
+}
+
+void
+main(int argc, char **argv)
+{
+	int fd, n;
+	uvlong pc, start, stop;
+	Fhdr fhdr;
+	Biobuf bout;
+	char buf[1024];
+	Map *text;
+
+	ARGBEGIN{
+	default:
+		usage();
+	}ARGEND
+
+	if(argc != 3)
+		usage();
+	start = strtoull(argv[1], 0, 16);
+	stop = strtoull(argv[2], 0, 16);
+
+	fd = open(argv[0], OREAD);
+	if(fd < 0)
+		sysfatal("open %s: %r", argv[0]);
+	if(crackhdr(fd, &fhdr) <= 0)
+		sysfatal("crackhdr: %r");
+	machbytype(fhdr.type);
+	if(syminit(fd, &fhdr) <= 0)
+		sysfatal("syminit: %r");
+	text = loadmap(nil, fd, &fhdr);
+	if(text == nil)
+		sysfatal("loadmap: %r");
+
+	Binit(&bout, 1, OWRITE);
+	for(pc=start; pc<stop; ) {
+		if(fileline(buf, sizeof buf, pc))
+			Bprint(&bout, "%s\n", buf);
+		buf[0] = '\0';
+		machdata->das(text, pc, 0, buf, sizeof buf);
+		Bprint(&bout, " %llx: %s\n", pc, buf);
+		n = machdata->instsize(text, pc);
+		if(n <= 0)
+			break;
+		pc += n;
+	}
+	Bflush(&bout);
+	exits(0);
+}
diff --git a/src/libmach/8db.c b/src/libmach/8db.c
index 3101e13730..ce1b4ddd76 100644
--- a/src/libmach/8db.c
+++ b/src/libmach/8db.c
@@ -1,11 +1,11 @@
 // Inferno libmach/8db.c
 // http://code.google.com/p/inferno-os/source/browse/utils/libmach/8db.c
 //
-// 	Copyright © 1994-1999 Lucent Technologies Inc.
-// 	Power PC support Copyright © 1995-2004 C H Forsyth (forsyth@terzarima.net).
-// 	Portions Copyright © 1997-1999 Vita Nuova Limited.
-// 	Portions Copyright © 2000-2007 Vita Nuova Holdings Limited (www.vitanuova.com).
-// 	Revisions Copyright © 2000-2004 Lucent Technologies Inc. and others.
+//	Copyright © 1994-1999 Lucent Technologies Inc.
+//	Power PC support Copyright © 1995-2004 C H Forsyth (forsyth@terzarima.net).
+//	Portions Copyright © 1997-1999 Vita Nuova Limited.
+//	Portions Copyright © 2000-2007 Vita Nuova Holdings Limited (www.vitanuova.com).
+//	Revisions Copyright © 2000-2004 Lucent Technologies Inc. and others.
 //	Portions Copyright © 2009 The Go Authors.  All rights reserved.
 //
 // Permission is hereby granted, free of charge, to any person obtaining a copy
@@ -2088,18 +2088,23 @@ immediate(Instr *ip, vlong val)
 static void
 pea(Instr *ip)
 {
+	int base;
+
+	base = ip->base;
+	if(base >= 0 && (ip->rex & REXB))
+		base += 8;
+
 	if (ip->mod == 3) {
 		if (ip->osize == 'B')
 			bprint(ip, (ip->rex & REXB? breg64: breg)[(uchar)ip->base]);
-		else if(ip->rex & REXB)
-			bprint(ip, "%s%s", ANAME(ip), reg[ip->base+8]);
 		else
-			bprint(ip, "%s%s", ANAME(ip), reg[(uchar)ip->base]);
+			bprint(ip, "%s%s", ANAME(ip), reg[base]);
 		return;
 	}
+
 	if (ip->segment)
 		bprint(ip, ip->segment);
-	if (ip->asize == 'E' && ip->base == SP)
+	if (ip->asize == 'E' && base == SP)
 		plocal(ip);
 	else {
 		if (ip->base < 0)
diff --git a/src/pkg/runtime/lock_futex.c b/src/pkg/runtime/lock_futex.c
index 6ec4aee7bf..b4465bff18 100644
--- a/src/pkg/runtime/lock_futex.c
+++ b/src/pkg/runtime/lock_futex.c
@@ -118,8 +118,12 @@ runtime·notewakeup(Note *n)
 void
 runtime·notesleep(Note *n)
 {
+	if(m->profilehz > 0)
+		runtime·setprof(false);
 	while(runtime·atomicload(&n->key) == 0)
 		runtime·futexsleep(&n->key, 0, -1);
+	if(m->profilehz > 0)
+		runtime·setprof(true);
 }
 
 void
@@ -135,14 +139,18 @@ runtime·notetsleep(Note *n, int64 ns)
 	if(runtime·atomicload(&n->key) != 0)
 		return;
 
+	if(m->profilehz > 0)
+		runtime·setprof(false);
 	deadline = runtime·nanotime() + ns;
 	for(;;) {
 		runtime·futexsleep(&n->key, 0, ns);
 		if(runtime·atomicload(&n->key) != 0)
-			return;
+			break;
 		now = runtime·nanotime();
 		if(now >= deadline)
-			return;
+			break;
 		ns = deadline - now;
 	}
+	if(m->profilehz > 0)
+		runtime·setprof(true);
 }
diff --git a/src/pkg/runtime/lock_sema.c b/src/pkg/runtime/lock_sema.c
index 28d2c3281e..1d9c37fdb6 100644
--- a/src/pkg/runtime/lock_sema.c
+++ b/src/pkg/runtime/lock_sema.c
@@ -154,7 +154,11 @@ runtime·notesleep(Note *n)
 		return;
 	}
 	// Queued.  Sleep.
+	if(m->profilehz > 0)
+		runtime·setprof(false);
 	runtime·semasleep(-1);
+	if(m->profilehz > 0)
+		runtime·setprof(true);
 }
 
 void
@@ -178,12 +182,16 @@ runtime·notetsleep(Note *n, int64 ns)
 		return;
 	}
 
+	if(m->profilehz > 0)
+		runtime·setprof(false);
 	deadline = runtime·nanotime() + ns;
 	for(;;) {
 		// Registered.  Sleep.
 		if(runtime·semasleep(ns) >= 0) {
 			// Acquired semaphore, semawakeup unregistered us.
 			// Done.
+			if(m->profilehz > 0)
+				runtime·setprof(true);
 			return;
 		}
 
@@ -196,6 +204,9 @@ runtime·notetsleep(Note *n, int64 ns)
 		ns = deadline - now;
 	}
 
+	if(m->profilehz > 0)
+		runtime·setprof(true);
+
 	// Deadline arrived.  Still registered.  Semaphore not acquired.
 	// Want to give up and return, but have to unregister first,
 	// so that any notewakeup racing with the return does not
diff --git a/src/pkg/runtime/os_darwin.h b/src/pkg/runtime/os_darwin.h
index 071a547177..eb5d2daa38 100644
--- a/src/pkg/runtime/os_darwin.h
+++ b/src/pkg/runtime/os_darwin.h
@@ -38,4 +38,7 @@ void	runtime·raisesigpipe(void);
 
 #define	NSIG 32
 #define	SI_USER	0  /* empirically true, but not what headers say */
+#define	SIG_BLOCK 1
+#define	SIG_UNBLOCK 2
 #define	SIG_SETMASK 3
+
diff --git a/src/pkg/runtime/os_freebsd.h b/src/pkg/runtime/os_freebsd.h
index da1d8de2eb..5e8de5434a 100644
--- a/src/pkg/runtime/os_freebsd.h
+++ b/src/pkg/runtime/os_freebsd.h
@@ -9,7 +9,6 @@ struct	sigaction;
 void	runtime·sigaction(int32, struct sigaction*, struct sigaction*);
 void	runtime·sigprocmask(Sigset *, Sigset *);
 void	runtime·setsig(int32, void(*)(int32, Siginfo*, void*, G*), bool);
-void	runtiem·setitimerval(int32, Itimerval*, Itimerval*);
 void	runtime·setitimer(int32, Itimerval*, Itimerval*);
 int32	runtime·sysctl(uint32*, uint32, byte*, uintptr*, byte*, uintptr);
 
diff --git a/src/pkg/runtime/os_netbsd.h b/src/pkg/runtime/os_netbsd.h
index 67c58ecb2a..4ecf78d882 100644
--- a/src/pkg/runtime/os_netbsd.h
+++ b/src/pkg/runtime/os_netbsd.h
@@ -12,7 +12,6 @@ void	runtime·sigaltstack(Sigaltstack*, Sigaltstack*);
 void	runtime·sigaction(int32, struct sigaction*, struct sigaction*);
 void	runtime·setsig(int32, void(*)(int32, Siginfo*, void*, G*), bool);
 void	runtime·sighandler(int32 sig, Siginfo *info, void *context, G *gp);
-void	runtime·setitimerval(int32, Itimerval*, Itimerval*);
 void	runtime·setitimer(int32, Itimerval*, Itimerval*);
 int32	runtime·sysctl(uint32*, uint32, byte*, uintptr*, byte*, uintptr);
 
diff --git a/src/pkg/runtime/os_openbsd.h b/src/pkg/runtime/os_openbsd.h
index 67c58ecb2a..4ecf78d882 100644
--- a/src/pkg/runtime/os_openbsd.h
+++ b/src/pkg/runtime/os_openbsd.h
@@ -12,7 +12,6 @@ void	runtime·sigaltstack(Sigaltstack*, Sigaltstack*);
 void	runtime·sigaction(int32, struct sigaction*, struct sigaction*);
 void	runtime·setsig(int32, void(*)(int32, Siginfo*, void*, G*), bool);
 void	runtime·sighandler(int32 sig, Siginfo *info, void *context, G *gp);
-void	runtime·setitimerval(int32, Itimerval*, Itimerval*);
 void	runtime·setitimer(int32, Itimerval*, Itimerval*);
 int32	runtime·sysctl(uint32*, uint32, byte*, uintptr*, byte*, uintptr);
 
diff --git a/src/pkg/runtime/pprof/pprof.go b/src/pkg/runtime/pprof/pprof.go
index 099bb6a92f..f67e8a8f9a 100644
--- a/src/pkg/runtime/pprof/pprof.go
+++ b/src/pkg/runtime/pprof/pprof.go
@@ -20,8 +20,8 @@ import (
 	"text/tabwriter"
 )
 
-// BUG(rsc): CPU profiling is broken on OS X, due to an Apple kernel bug.
-// For details, see http://code.google.com/p/go/source/detail?r=35b716c94225.
+// BUG(rsc): A bug in the OS X Snow Leopard 64-bit kernel prevents
+// CPU profiling from giving accurate results on that system.
 
 // A Profile is a collection of stack traces showing the call sequences
 // that led to instances of a particular event, such as allocation.
@@ -156,7 +156,7 @@ func (p *Profile) Count() int {
 }
 
 // Add adds the current execution stack to the profile, associated with value.
-// Add stores value in an internal map, so value must be suitable for use as 
+// Add stores value in an internal map, so value must be suitable for use as
 // a map key and will not be garbage collected until the corresponding
 // call to Remove.  Add panics if the profile already contains a stack for value.
 //
diff --git a/src/pkg/runtime/pprof/pprof_test.go b/src/pkg/runtime/pprof/pprof_test.go
index 5f128c01cf..994ec9dde4 100644
--- a/src/pkg/runtime/pprof/pprof_test.go
+++ b/src/pkg/runtime/pprof/pprof_test.go
@@ -7,6 +7,7 @@ package pprof_test
 import (
 	"bytes"
 	"hash/crc32"
+	"os/exec"
 	"runtime"
 	. "runtime/pprof"
 	"strings"
@@ -17,8 +18,15 @@ import (
 func TestCPUProfile(t *testing.T) {
 	switch runtime.GOOS {
 	case "darwin":
-		// see Apple Bug Report #9177434 (copied into change description)
-		return
+		out, err := exec.Command("uname", "-a").CombinedOutput()
+		if err != nil {
+			t.Fatal(err)
+		}
+		vers := string(out)
+		t.Logf("uname -a: %v", vers)
+		if strings.Contains(vers, "Darwin Kernel Version 10.8.0") && strings.Contains(vers, "root:xnu-1504.15.3~1/RELEASE_X86_64") {
+			t.Logf("skipping test on known-broken kernel (64-bit Snow Leopard)")
+		}
 	case "plan9":
 		// unimplemented
 		return
diff --git a/src/pkg/runtime/proc.c b/src/pkg/runtime/proc.c
index d94bec8855..ddac048a00 100644
--- a/src/pkg/runtime/proc.c
+++ b/src/pkg/runtime/proc.c
@@ -338,7 +338,7 @@ mcommoninit(M *m)
 		m->mcache = runtime·allocmcache();
 
 	runtime·callers(1, m->createstack, nelem(m->createstack));
-	
+
 	// Add to runtime·allm so garbage collector doesn't free m
 	// when it is just in a register or thread-local storage.
 	m->alllink = runtime·allm;
@@ -728,7 +728,6 @@ runtime·mstart(void)
 	// so other calls can reuse this stack space.
 	runtime·gosave(&m->g0->sched);
 	m->g0->sched.pc = (void*)-1;  // make sure it is never used
-
 	runtime·asminit();
 	runtime·minit();
 	schedule(nil);
@@ -916,6 +915,9 @@ runtime·entersyscall(void)
 {
 	uint32 v;
 
+	if(m->profilehz > 0)
+		runtime·setprof(false);
+
 	// Leave SP around for gc and traceback.
 	runtime·gosave(&g->sched);
 	g->gcsp = g->sched.sp;
@@ -979,6 +981,9 @@ runtime·exitsyscall(void)
 		// Garbage collector isn't running (since we are),
 		// so okay to clear gcstack.
 		g->gcstack = nil;
+
+		if(m->profilehz > 0)
+			runtime·setprof(true);
 		return;
 	}
 
diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index 3b0f505e72..f2669fdb7e 100644
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -730,3 +730,13 @@ bool	runtime·showframe(Func*);
 void	runtime·ifaceE2I(struct InterfaceType*, Eface, Iface*);
 
 uintptr	runtime·memlimit(void);
+
+// If appropriate, ask the operating system to control whether this
+// thread should receive profiling signals.  This is only necessary on OS X.
+// An operating system should not deliver a profiling signal to a
+// thread that is not actually executing (what good is that?), but that's
+// what OS X prefers to do.  When profiling is turned on, we mask
+// away the profiling signal when threads go to sleep, so that OS X
+// is forced to deliver the signal to a thread that's actually running.
+// This is a no-op on other systems.
+void	runtime·setprof(bool);
diff --git a/src/pkg/runtime/signal_darwin_386.c b/src/pkg/runtime/signal_darwin_386.c
index 1844f68a63..9e986352b4 100644
--- a/src/pkg/runtime/signal_darwin_386.c
+++ b/src/pkg/runtime/signal_darwin_386.c
@@ -40,7 +40,8 @@ runtime·sighandler(int32 sig, Siginfo *info, void *context, G *gp)
 	r = &mc->ss;
 
 	if(sig == SIGPROF) {
-		runtime·sigprof((uint8*)r->eip, (uint8*)r->esp, nil, gp);
+		if(gp != m->g0 && gp != m->gsignal)
+			runtime·sigprof((uint8*)r->eip, (uint8*)r->esp, nil, gp);
 		return;
 	}
 
@@ -58,7 +59,7 @@ runtime·sighandler(int32 sig, Siginfo *info, void *context, G *gp)
 			if(pc[0] == 0xF6 || pc[0] == 0xF7)
 				info->si_code = FPE_INTDIV;
 		}
-		
+
 		// Make it look like a call to the signal func.
 		// Have to pass arguments out of band since
 		// augmenting the stack frame would break
diff --git a/src/pkg/runtime/signal_darwin_amd64.c b/src/pkg/runtime/signal_darwin_amd64.c
index 32c73081c1..d9c5f48e7c 100644
--- a/src/pkg/runtime/signal_darwin_amd64.c
+++ b/src/pkg/runtime/signal_darwin_amd64.c
@@ -48,7 +48,8 @@ runtime·sighandler(int32 sig, Siginfo *info, void *context, G *gp)
 	r = &mc->ss;
 
 	if(sig == SIGPROF) {
-		runtime·sigprof((uint8*)r->rip, (uint8*)r->rsp, nil, gp);
+		if(gp != m->g0 && gp != m->gsignal)
+			runtime·sigprof((uint8*)r->rip, (uint8*)r->rsp, nil, gp);
 		return;
 	}
 
@@ -68,7 +69,7 @@ runtime·sighandler(int32 sig, Siginfo *info, void *context, G *gp)
 			if(pc[0] == 0xF6 || pc[0] == 0xF7)
 				info->si_code = FPE_INTDIV;
 		}
-		
+
 		// Make it look like a call to the signal func.
 		// Have to pass arguments out of band since
 		// augmenting the stack frame would break
@@ -77,7 +78,7 @@ runtime·sighandler(int32 sig, Siginfo *info, void *context, G *gp)
 		gp->sigcode0 = info->si_code;
 		gp->sigcode1 = (uintptr)info->si_addr;
 		gp->sigpc = r->rip;
-		
+
 		// Only push runtime·sigpanic if r->rip != 0.
 		// If r->rip == 0, probably panicked because of a
 		// call to a nil func.  Not pushing that onto sp will
@@ -92,7 +93,7 @@ runtime·sighandler(int32 sig, Siginfo *info, void *context, G *gp)
 		r->rip = (uintptr)runtime·sigpanic;
 		return;
 	}
-	
+
 	if(info->si_code == SI_USER || (t->flags & SigNotify))
 		if(runtime·sigsend(sig))
 			return;
diff --git a/src/pkg/runtime/signal_unix.c b/src/pkg/runtime/signal_unix.c
index 13708415b9..0b9d2a55a1 100644
--- a/src/pkg/runtime/signal_unix.c
+++ b/src/pkg/runtime/signal_unix.c
@@ -55,17 +55,17 @@ void
 runtime·resetcpuprofiler(int32 hz)
 {
 	Itimerval it;
-	
+
 	runtime·memclr((byte*)&it, sizeof it);
 	if(hz == 0) {
 		runtime·setitimer(ITIMER_PROF, &it, nil);
-		runtime·setsig(SIGPROF, SIG_IGN, true);
+		runtime·setprof(false);
 	} else {
-		runtime·setsig(SIGPROF, runtime·sighandler, true);
 		it.it_interval.tv_sec = 0;
 		it.it_interval.tv_usec = 1000000 / hz;
 		it.it_value = it.it_interval;
 		runtime·setitimer(ITIMER_PROF, &it, nil);
+		runtime·setprof(true);
 	}
 	m->profilehz = hz;
 }
diff --git a/src/pkg/runtime/sys_darwin_386.s b/src/pkg/runtime/sys_darwin_386.s
index e235a8473d..c2dab8931c 100644
--- a/src/pkg/runtime/sys_darwin_386.s
+++ b/src/pkg/runtime/sys_darwin_386.s
@@ -100,14 +100,14 @@ TEXT runtime·nanotime(SB), 7, $32
 	IMULL	$1000, BX
 	ADDL	BX, AX
 	ADCL	$0, DX
-	
+
 	MOVL	ret+0(FP), DI
 	MOVL	AX, 0(DI)
 	MOVL	DX, 4(DI)
 	RET
 
 TEXT runtime·sigprocmask(SB),7,$0
-	MOVL	$48, AX
+	MOVL	$329, AX  // pthread_sigmask (on OS X, sigprocmask==entire process)
 	INT	$0x80
 	JAE	2(PC)
 	CALL	runtime·notok(SB)
diff --git a/src/pkg/runtime/sys_darwin_amd64.s b/src/pkg/runtime/sys_darwin_amd64.s
index 13882c8524..4b215d04d4 100644
--- a/src/pkg/runtime/sys_darwin_amd64.s
+++ b/src/pkg/runtime/sys_darwin_amd64.s
@@ -96,7 +96,7 @@ TEXT runtime·sigprocmask(SB),7,$0
 	MOVL	8(SP), DI
 	MOVQ	16(SP), SI
 	MOVQ	24(SP), DX
-	MOVL	$(0x2000000+48), AX	// syscall entry
+	MOVL	$(0x2000000+329), AX  // pthread_sigmask (on OS X, sigprocmask==entire process)
 	SYSCALL
 	JCC	2(PC)
 	CALL	runtime·notok(SB)
diff --git a/src/pkg/runtime/thread_darwin.c b/src/pkg/runtime/thread_darwin.c
index d170dfb3d3..556fb67e84 100644
--- a/src/pkg/runtime/thread_darwin.c
+++ b/src/pkg/runtime/thread_darwin.c
@@ -11,6 +11,7 @@ extern SigTab runtime·sigtab[];
 
 static Sigset sigset_all = ~(Sigset)0;
 static Sigset sigset_none;
+static Sigset sigset_prof = 1<<(SIGPROF-1);
 
 static void
 unimplemented(int8 *name)
@@ -23,7 +24,14 @@ unimplemented(int8 *name)
 int32
 runtime·semasleep(int64 ns)
 {
-	return runtime·mach_semacquire(m->waitsema, ns);
+	int32 v;
+
+	if(m->profilehz > 0)
+		runtime·setprof(false);
+	v = runtime·mach_semacquire(m->waitsema, ns);
+	if(m->profilehz > 0)
+		runtime·setprof(true);
+	return v;
 }
 
 void
@@ -84,7 +92,7 @@ runtime·newosproc(M *m, G *g, void *stk, void (*fn)(void))
 	runtime·sigprocmask(SIG_SETMASK, &sigset_all, &oset);
 	errno = runtime·bsdthread_create(stk, m, g, fn);
 	runtime·sigprocmask(SIG_SETMASK, &oset, nil);
-	
+
 	if(errno < 0) {
 		runtime·printf("runtime: failed to create new OS thread (have %d already; errno=%d)\n", runtime·mcount(), -errno);
 		runtime·throw("runtime.newosproc");
@@ -98,7 +106,11 @@ runtime·minit(void)
 	// Initialize signal handling.
 	m->gsignal = runtime·malg(32*1024);	// OS X wants >=8K, Linux >=2K
 	runtime·signalstack(m->gsignal->stackguard - StackGuard, 32*1024);
-	runtime·sigprocmask(SIG_SETMASK, &sigset_none, nil);
+
+	if(m->profilehz > 0)
+		runtime·sigprocmask(SIG_SETMASK, &sigset_none, nil);
+	else
+		runtime·sigprocmask(SIG_SETMASK, &sigset_prof, nil);
 }
 
 // Mach IPC, to get at semaphores
@@ -434,3 +446,34 @@ runtime·memlimit(void)
 	// the limit.
 	return 0;
 }
+
+// NOTE(rsc): On OS X, when the CPU profiling timer expires, the SIGPROF
+// signal is not guaranteed to be sent to the thread that was executing to
+// cause it to expire.  It can and often does go to a sleeping thread, which is
+// not interesting for our profile.  This is filed Apple Bug Report #9177434,
+// copied to http://code.google.com/p/go/source/detail?r=35b716c94225.
+// To work around this bug, we disable receipt of the profiling signal on
+// a thread while in blocking system calls.  This forces the kernel to deliver
+// the profiling signal to an executing thread.
+//
+// The workaround fails on OS X machines using a 64-bit Snow Leopard kernel.
+// In that configuration, the kernel appears to want to deliver SIGPROF to the
+// sleeping threads regardless of signal mask and, worse, does not deliver
+// the signal until the thread wakes up on its own.
+//
+// If necessary, we can switch to using ITIMER_REAL for OS X and handle
+// the kernel-generated SIGALRM by generating our own SIGALRMs to deliver
+// to all the running threads.  SIGALRM does not appear to be affected by
+// the 64-bit Snow Leopard bug.  However, as of this writing Mountain Lion
+// is in preview, making Snow Leopard two versions old, so it is unclear how
+// much effort we need to spend on one buggy kernel.
+
+// Control whether profiling signal can be delivered to this thread.
+void
+runtime·setprof(bool on)
+{
+	if(on)
+		runtime·sigprocmask(SIG_UNBLOCK, &sigset_prof, nil);
+	else
+		runtime·sigprocmask(SIG_BLOCK, &sigset_prof, nil);
+}
diff --git a/src/pkg/runtime/thread_freebsd.c b/src/pkg/runtime/thread_freebsd.c
index 7871827a97..77e8bb3dac 100644
--- a/src/pkg/runtime/thread_freebsd.c
+++ b/src/pkg/runtime/thread_freebsd.c
@@ -189,3 +189,9 @@ runtime·memlimit(void)
 
 	return rl.rlim_cur - used;
 }
+
+void
+runtime·setprof(bool on)
+{
+	USED(on);
+}
diff --git a/src/pkg/runtime/thread_linux.c b/src/pkg/runtime/thread_linux.c
index d406a71240..6b428440e0 100644
--- a/src/pkg/runtime/thread_linux.c
+++ b/src/pkg/runtime/thread_linux.c
@@ -228,7 +228,7 @@ runtime·memlimit(void)
 	Rlimit rl;
 	extern byte text[], end[];
 	uintptr used;
-	
+
 	if(runtime·getrlimit(RLIMIT_AS, &rl) != 0)
 		return 0;
 	if(rl.rlim_cur >= 0x7fffffff)
@@ -249,3 +249,9 @@ runtime·memlimit(void)
 
 	return rl.rlim_cur - used;
 }
+
+void
+runtime·setprof(bool on)
+{
+	USED(on);
+}
diff --git a/src/pkg/runtime/thread_netbsd.c b/src/pkg/runtime/thread_netbsd.c
index 7d14e5c68b..62e133c449 100644
--- a/src/pkg/runtime/thread_netbsd.c
+++ b/src/pkg/runtime/thread_netbsd.c
@@ -207,3 +207,9 @@ runtime·memlimit(void)
 {
 	return 0;
 }
+
+void
+runtime·setprof(bool on)
+{
+	USED(on);
+}
diff --git a/src/pkg/runtime/thread_openbsd.c b/src/pkg/runtime/thread_openbsd.c
index 704d95a3c6..bee0c5755f 100644
--- a/src/pkg/runtime/thread_openbsd.c
+++ b/src/pkg/runtime/thread_openbsd.c
@@ -207,3 +207,9 @@ runtime·memlimit(void)
 {
 	return 0;
 }
+
+void
+runtime·setprof(bool on)
+{
+	USED(on);
+}
diff --git a/src/pkg/runtime/thread_plan9.c b/src/pkg/runtime/thread_plan9.c
index 7d5c38fc9a..aaed5050bb 100644
--- a/src/pkg/runtime/thread_plan9.c
+++ b/src/pkg/runtime/thread_plan9.c
@@ -87,7 +87,7 @@ runtime·nanotime(void)
 	// The naïve implementation (without the cached
 	// file descriptor) is roughly four times slower
 	// in 9vx on a 2.16 GHz Intel Core 2 Duo.
-	
+
 	if(fd < 0 && (fd = runtime·open((byte*)"/dev/bintime", OREAD|OCEXEC)) < 0)
 		return 0;
 	if(runtime·pread(fd, b, sizeof b, 0) != sizeof b)
@@ -241,3 +241,9 @@ runtime·memlimit(void)
 {
 	return 0;
 }
+
+void
+runtime·setprof(bool on)
+{
+	USED(on);
+}
diff --git a/src/pkg/runtime/thread_windows.c b/src/pkg/runtime/thread_windows.c
index 8feac9711d..8a448bc37c 100644
--- a/src/pkg/runtime/thread_windows.c
+++ b/src/pkg/runtime/thread_windows.c
@@ -226,7 +226,7 @@ void
 time·now(int64 sec, int32 usec)
 {
 	int64 ns;
-	
+
 	ns = runtime·nanotime();
 	sec = ns / 1000000000LL;
 	usec = ns - sec * 1000000000LL;
@@ -431,3 +431,9 @@ runtime·memlimit(void)
 {
 	return 0;
 }
+
+void
+runtime·setprof(bool on)
+{
+	USED(on);
+}