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cmd/compile: generate load without DS relocation for go.string on ppc64le
Due to some recent optimizations related to the compare instruction, DS-form load instructions started to be used to load 8-byte go.strings. This can cause link time errors if the go.string is not aligned to 4 bytes. For DS-form instructions, the value in the offset field must be a multiple of 4. If the offset is known at the time the rules are processed, a DS-form load will not be chosen. But for go.strings, the offset is not known at that time, but a relocation is generated indicating that the linker should fill in the DS relocation. When the linker tries to fill in the relocation, if the offset is not aligned properly, a link error will occur. To fix this, when loading a go.string using MOVDload, the full address of the go.string is generated and loaded into the base register. Then the go.string is loaded with a 0 offset field. Added a testcase that reproduces this problem. Fixes #24799 Change-Id: I6a154e8e1cba64eae290be0fbcb608b75884ecdd Reviewed-on: https://go-review.googlesource.com/107855 Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: David Chase <drchase@google.com>
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@ -11,6 +11,7 @@ import (
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"cmd/internal/obj"
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"cmd/internal/obj"
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"cmd/internal/obj/ppc64"
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"cmd/internal/obj/ppc64"
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"math"
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"math"
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"strings"
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)
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)
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// iselOp encodes mapping of comparison operations onto ISEL operands
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// iselOp encodes mapping of comparison operations onto ISEL operands
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@ -680,7 +681,42 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
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p.To.Reg = v.Reg()
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p.To.Reg = v.Reg()
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p.To.Type = obj.TYPE_REG
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p.To.Type = obj.TYPE_REG
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case ssa.OpPPC64MOVDload, ssa.OpPPC64MOVWload, ssa.OpPPC64MOVHload, ssa.OpPPC64MOVWZload, ssa.OpPPC64MOVBZload, ssa.OpPPC64MOVHZload:
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case ssa.OpPPC64MOVDload:
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// MOVDload uses a DS instruction which requires the offset value of the data to be a multiple of 4.
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// For offsets known at compile time, a MOVDload won't be selected, but in the case of a go.string,
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// the offset is not known until link time. If the load of a go.string uses relocation for the
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// offset field of the instruction, and if the offset is not aligned to 4, then a link error will occur.
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// To avoid this problem, the full address of the go.string is computed and loaded into the base register,
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// and that base register is used for the MOVDload using a 0 offset. This problem can only occur with
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// go.string types because other types will have proper alignment.
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gostring := false
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switch n := v.Aux.(type) {
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case *obj.LSym:
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gostring = strings.HasPrefix(n.Name, "go.string.")
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}
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if gostring {
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// Generate full addr of the go.string const
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// including AuxInt
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p := s.Prog(ppc64.AMOVD)
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p.From.Type = obj.TYPE_ADDR
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p.From.Reg = v.Args[0].Reg()
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gc.AddAux(&p.From, v)
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p.To.Type = obj.TYPE_REG
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p.To.Reg = v.Reg()
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// Load go.string using 0 offset
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p = s.Prog(v.Op.Asm())
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p.From.Type = obj.TYPE_MEM
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p.From.Reg = v.Reg()
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p.To.Type = obj.TYPE_REG
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p.To.Reg = v.Reg()
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break
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}
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// Not a go.string, generate a normal load
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fallthrough
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case ssa.OpPPC64MOVWload, ssa.OpPPC64MOVHload, ssa.OpPPC64MOVWZload, ssa.OpPPC64MOVBZload, ssa.OpPPC64MOVHZload:
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p := s.Prog(v.Op.Asm())
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p := s.Prog(v.Op.Asm())
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p.From.Type = obj.TYPE_MEM
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p.From.Type = obj.TYPE_MEM
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p.From.Reg = v.Args[0].Reg()
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p.From.Reg = v.Args[0].Reg()
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58
test/fixedbugs/issue24799.go
Normal file
58
test/fixedbugs/issue24799.go
Normal file
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@ -0,0 +1,58 @@
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// run
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// Copyright 2018 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Loads of 8 byte go.strings cannot use DS relocation
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// in case the alignment is not a multiple of 4.
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package main
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import (
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"fmt"
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)
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type Level string
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// The following are all go.strings. A link time error can
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// occur if an 8 byte load is used to load a go.string that is
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// not aligned to 4 bytes due to the type of relocation that
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// is generated for the instruction. A fix was made to avoid
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// generating an instruction with DS relocation for go.strings
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// since their alignment is not known until link time.
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// This problem only affects go.string since other types have
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// correct alignment.
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const (
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LevelBad Level = "badvals"
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LevelNone Level = "No"
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LevelMetadata Level = "Metadata"
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LevelRequest Level = "Request"
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LevelRequestResponse Level = "RequestResponse"
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)
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func ordLevel(l Level) int {
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switch l {
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case LevelMetadata:
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return 1
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case LevelRequest:
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return 2
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case LevelRequestResponse:
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return 3
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default:
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return 0
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}
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}
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//go:noinline
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func test(l Level) {
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if ordLevel(l) < ordLevel(LevelMetadata) {
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fmt.Printf("OK\n")
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}
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}
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func main() {
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test(LevelMetadata)
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}
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