cmd/compile: during regalloc, fixedreg values are always available

randall77 · randall77 · commit 9bbea0f21a45 · 2025-08-13T14:55:05.000-07:00
It is ok to clobber registers that have a copy of a fixedreg value, as that value is always available in its original location later if we need it. (See 14 lines below the change.) This CL will fix the regalloc infinite loop that CL 678620 introduced. That CL requests that the stack pointer value be materialized in a non-stack-pointer register, which is atypical. That condition triggered the infinite loop that this CL fixes. The infinite loop is the compiler trying to reuse that non-stack-pointer register for something else, but then refusing to give it up because it thought that non-stack-pointer register held the last copy of the original SP value. Change-Id: Id604d0937fb9d3753ee273bf1917753d3ef2d5d7 Reviewed-on: https://go-review.googlesource.com/c/go/+/696035 Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Keith Randall <khr@google.com> LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
diff --git a/src/cmd/compile/internal/ssa/regalloc.go b/src/cmd/compile/internal/ssa/regalloc.go
@@ -2476,7 +2476,7 @@ func (e *edgeState) processDest(loc Location, vid ID, splice **Value, pos src.XP
 	}
 
 	// Check if we're allowed to clobber the destination location.
-	if len(e.cache[occupant.vid]) == 1 && !e.s.values[occupant.vid].rematerializeable {
+	if len(e.cache[occupant.vid]) == 1 && !e.s.values[occupant.vid].rematerializeable && !opcodeTable[e.s.orig[occupant.vid].Op].fixedReg {
 		// We can't overwrite the last copy
 		// of a value that needs to survive.
 		return false

Original file line number	Diff line number	Diff line change
`@@ -2476,7 +2476,7 @@ func (e edgeState) processDest(loc Location, vid ID, splice *Value, pos src.XP`
`2476`	`2476`	`}`
`2477`	`2477`
`2478`	`2478`	`// Check if we're allowed to clobber the destination location.`
`2479`		`- if len(e.cache[occupant.vid]) == 1 && !e.s.values[occupant.vid].rematerializeable {`
	`2479`	`+ if len(e.cache[occupant.vid]) == 1 && !e.s.values[occupant.vid].rematerializeable && !opcodeTable[e.s.orig[occupant.vid].Op].fixedReg {`
`2480`	`2480`	`// We can't overwrite the last copy`
`2481`	`2481`	`// of a value that needs to survive.`
`2482`	`2482`	`return false`