cmd/compile: fix mips64 instruction UNPREDICTABLE bug

Replace addu with a sll instruction with a definite behavior (sll will discard the upper 32 bits of the 64 bits, then do sign extensions, with certain behavior). It won't have any UNPREDICTABLE expectation.

Fixes #30459

Change-Id: Id79085c28c5cc4f86939b4ef08ef4bff46077c45
GitHub-Last-Rev: 03569796a9a64ed6c7d56a5bca966fc62c89b4ff
GitHub-Pull-Request: golang/go#30461
Reviewed-on: https://go-review.googlesource.com/c/go/+/164758
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

diff --git a/src/cmd/internal/obj/mips/asm0.go b/src/cmd/internal/obj/mips/asm0.go
index 458e071e4767e20102429a150840152a70f311f2..c117269c35ad0b323f30b8bc54aa286626050a47 100644 (file)
--- a/src/cmd/internal/obj/mips/asm0.go
+++ b/src/cmd/internal/obj/mips/asm0.go
@@ -1120,9 +1120,11 @@ func (c *ctxt0) asmout(p *obj.Prog, o *Optab, out []uint32) {
        case 1: /* mov r1,r2 ==> OR r1,r0,r2 */
                a := AOR
                if p.As == AMOVW && c.ctxt.Arch.Family == sys.MIPS64 {
-                       a = AADDU // sign-extended to high 32 bits
+                       // on MIPS64, most of the 32-bit instructions have unpredictable behavior,
+                       // but SLL is special that the result is always sign-extended to 64-bit.
+                       a = ASLL
                }
-               o1 = OP_RRR(c.oprrr(a), uint32(REGZERO), uint32(p.From.Reg), uint32(p.To.Reg))
+               o1 = OP_RRR(c.oprrr(a), uint32(p.From.Reg), uint32(REGZERO), uint32(p.To.Reg))
 
        case 2: /* add/sub r1,[r2],r3 */
                r := int(p.Reg)