undo CL 102820043 / b0ce6dbafc18

Breaks 386 and arm builds.
The obvious reason is that this CL only edited 6g/gsubr.c
and failed to edit 5g/gsubr.c and 8g/gsubr.c.
However, the obvious CL applying the same edit to those
files (CL 101900043) causes mysterious build failures
in various of the standard package tests, usually involving
reflect. Something deep and subtle is broken but only on
the 32-bit systems.

Undo this CL for now.

««« original CL description
cmd/gc: fix x=x crash

The 'nodarg' function is used to obtain a Node*
representing a function argument or result.
It returned a brand new Node*, but that violates
the guarantee in most places in the compiler that
two Node*s refer to the same variable if and only if
they are the same Node* pointer. Reestablish that
invariant by making nodarg return a preexisting
named variable if present.

Having fixed that, avoid any copy during x=x in
componentgen, because the VARDEF we emit
before the copy marks the lhs x as dead incorrectly.

The change in walk.c avoids modifying the result
of nodarg. This was the only place in the compiler
that did so.

Fixes #8097.

LGTM=r, khr
R=golang-codereviews, r, khr
CC=golang-codereviews, iant
https://golang.org/cl/102820043
»»»

TBR=r
CC=golang-codereviews, khr
https://golang.org/cl/95660043

src/cmd/5g/cgen.c

@@ -1490,7 +1490,6 @@ sgen(Node *n, Node *res, int64 w)
 	}
 	if(osrc%align != 0 || odst%align != 0)
 		fatal("sgen: unaligned offset src %d or dst %d (align %d)", osrc, odst, align);
-
 	// if we are copying forward on the stack and
 	// the src and dst overlap, then reverse direction
 	dir = align;
@@ -1675,13 +1674,6 @@ componentgen(Node *nr, Node *nl)
 		freer = 1;
 	}
 
-	// nl and nr are 'cadable' which basically means they are names (variables) now.
-	// If they are the same variable, don't generate any code, because the
-	// VARDEF we generate will mark the old value as dead incorrectly.
-	// (And also the assignments are useless.)
-	if(nr != N && nl->op == ONAME && nr->op == ONAME && nl == nr)
-		goto yes;
-
 	switch(nl->type->etype) {
 	case TARRAY:
 		if(nl->op == ONAME)

src/cmd/6g/cgen.c

@@ -1585,13 +1585,6 @@ componentgen(Node *nr, Node *nl)
 			freer = 1;
 		}
 	}
-	
-	// nl and nr are 'cadable' which basically means they are names (variables) now.
-	// If they are the same variable, don't generate any code, because the
-	// VARDEF we generate will mark the old value as dead incorrectly.
-	// (And also the assignments are useless.)
-	if(nr != N && nl->op == ONAME && nr->op == ONAME && nl == nr)
-		goto yes;
 
 	switch(nl->type->etype) {
 	case TARRAY:

src/cmd/6g/gsubr.c

@@ -462,7 +462,6 @@ Node*
 nodarg(Type *t, int fp)
 {
 	Node *n;
-	NodeList *l;
 	Type *first;
 	Iter savet;
 
@@ -483,14 +482,6 @@ nodarg(Type *t, int fp)
 
 	if(t->etype != TFIELD)
 		fatal("nodarg: not field %T", t);
-	
-	if(fp == 1) {
-		for(l=curfn->dcl; l; l=l->next) {
-			n = l->n;
-			if((n->class == PPARAM || n->class == PPARAMOUT) && !isblanksym(t->sym) && n->sym == t->sym)
-				return n;
-		}
-	}
 
 	n = nod(ONAME, N, N);
 	n->type = t->type;

src/cmd/8g/cgen.c

@@ -1397,13 +1397,6 @@ componentgen(Node *nr, Node *nl)
 		}
 	}
 
-	// nl and nr are 'cadable' which basically means they are names (variables) now.
-	// If they are the same variable, don't generate any code, because the
-	// VARDEF we generate will mark the old value as dead incorrectly.
-	// (And also the assignments are useless.)
-	if(nr != N && nl->op == ONAME && nr->op == ONAME && nl == nr)
-		goto yes;
-
 	switch(nl->type->etype) {
 	case TARRAY:
 		if(nl->op == ONAME)

src/cmd/gc/walk.c

@@ -1652,8 +1652,7 @@ ascompatte(int op, Node *call, int isddd, Type **nl, NodeList *lr, int fp, NodeL
 		// optimization - can do block copy
 		if(eqtypenoname(r->type, *nl)) {
 			a = nodarg(*nl, fp);
-			r = nod(OCONVNOP, r, N);
-			r->type = a->type;
+			a->type = r->type;
 			nn = list1(convas(nod(OAS, a, r), init));
 			goto ret;
 		}

test/live.go

@@ -564,29 +564,3 @@ func f38(b bool) {
 	}
 	println()
 }
-
-// issue 8097: mishandling of x = x during return.
-
-func f39() (x []int) {
-	x = []int{1}
-	println() // ERROR "live at call to printnl: x"
-	return x
-}
-
-func f39a() (x []int) {
-	x = []int{1}
-	println() // ERROR "live at call to printnl: x"
-	return
-}
-
-func f39b() (x [10]*int) {
-	x = [10]*int{new(int)} // ERROR "live at call to new: x"
-	println() // ERROR "live at call to printnl: x"
-	return x
-}
-
-func f39c() (x [10]*int) {
-	x = [10]*int{new(int)} // ERROR "live at call to new: x"
-	println() // ERROR "live at call to printnl: x"
-	return
-}