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>

src/cmd/compile/internal/ppc64/ssa.go

@@ -11,6 +11,7 @@ import (
 	"cmd/internal/obj"
 	"cmd/internal/obj/ppc64"
 	"math"
+	"strings"
 )
 
 // iselOp encodes mapping of comparison operations onto ISEL operands
@@ -680,7 +681,42 @@ func ssaGenValue(s *gc.SSAGenState, v *ssa.Value) {
 		p.To.Reg = v.Reg()
 		p.To.Type = obj.TYPE_REG
 
-	case ssa.OpPPC64MOVDload, ssa.OpPPC64MOVWload, ssa.OpPPC64MOVHload, ssa.OpPPC64MOVWZload, ssa.OpPPC64MOVBZload, ssa.OpPPC64MOVHZload:
+	case ssa.OpPPC64MOVDload:
+
+		// MOVDload uses a DS instruction which requires the offset value of the data to be a multiple of 4.
+		// For offsets known at compile time, a MOVDload won't be selected, but in the case of a go.string,
+		// the offset is not known until link time. If the load of a go.string uses relocation for the
+		// offset field of the instruction, and if the offset is not aligned to 4, then a link error will occur.
+		// To avoid this problem, the full address of the go.string is computed and loaded into the base register,
+		// and that base register is used for the MOVDload using a 0 offset. This problem can only occur with
+		// go.string types because other types will have proper alignment.
+
+		gostring := false
+		switch n := v.Aux.(type) {
+		case *obj.LSym:
+			gostring = strings.HasPrefix(n.Name, "go.string.")
+		}
+		if gostring {
+			// Generate full addr of the go.string const
+			// including AuxInt
+			p := s.Prog(ppc64.AMOVD)
+			p.From.Type = obj.TYPE_ADDR
+			p.From.Reg = v.Args[0].Reg()
+			gc.AddAux(&p.From, v)
+			p.To.Type = obj.TYPE_REG
+			p.To.Reg = v.Reg()
+			// Load go.string using 0 offset
+			p = s.Prog(v.Op.Asm())
+			p.From.Type = obj.TYPE_MEM
+			p.From.Reg = v.Reg()
+			p.To.Type = obj.TYPE_REG
+			p.To.Reg = v.Reg()
+			break
+		}
+		// Not a go.string, generate a normal load
+		fallthrough
+
+	case ssa.OpPPC64MOVWload, ssa.OpPPC64MOVHload, ssa.OpPPC64MOVWZload, ssa.OpPPC64MOVBZload, ssa.OpPPC64MOVHZload:
 		p := s.Prog(v.Op.Asm())
 		p.From.Type = obj.TYPE_MEM
 		p.From.Reg = v.Args[0].Reg()

test/fixedbugs/issue24799.go

+// run
+
+// Copyright 2018 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.
+
+// Loads of 8 byte go.strings cannot use DS relocation
+// in case the alignment is not a multiple of 4.
+
+package main
+
+import (
+        "fmt"
+)
+
+type Level string
+
+// The following are all go.strings. A link time error can
+// occur if an 8 byte load is used to load a go.string that is
+// not aligned to 4 bytes due to the type of relocation that
+// is generated for the instruction. A fix was made to avoid
+// generating an instruction with DS relocation for go.strings
+// since their alignment is not known until link time. 
+
+// This problem only affects go.string since other types have
+// correct alignment.
+
+const (
+        LevelBad Level = "badvals"
+        LevelNone Level = "No"
+        LevelMetadata Level = "Metadata"
+        LevelRequest Level = "Request"
+        LevelRequestResponse Level = "RequestResponse"
+)
+
+func ordLevel(l Level) int {
+        switch l {
+        case LevelMetadata:
+                return 1
+        case LevelRequest:
+                return 2
+        case LevelRequestResponse:
+                return 3
+        default:
+                return 0
+        }
+}
+
+//go:noinline
+func test(l Level) {
+        if ordLevel(l) < ordLevel(LevelMetadata) {
+                fmt.Printf("OK\n")
+        }
+}
+
+func main() {
+        test(LevelMetadata)
+}