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cmd/compile: learn transitive proofs for safe unsigned subs

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I've split this into it's own CL to make git bisect more effective.

Change-Id: I436ff21a3e2362b3924de25a458534eb9947e013
Reviewed-on: https://go-review.googlesource.com/c/go/+/685821
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
This commit is contained in:
Jorropo 2025-07-04 09:26:38 +02:00 • committed by Gopher Robot
parent d574856482
commit f32cf8e4b0
2 changed files with 76 additions and 0 deletions
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15
src/cmd/compile/internal/ssa/prove.go
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@ -2164,6 +2164,10 @@ func signedAddOverflowsOrUnderflows(a, b int64, t *types.Type) bool {
}
}
func unsignedSubUnderflows(a, b uint64) bool {
return a < b
}
func addLocalFacts(ft *factsTable, b *Block) {
// Propagate constant ranges among values in this block.
// We do this before the second loop so that we have the
@ -2226,6 +2230,17 @@ func addLocalFacts(ft *factsTable, b *Block) {
}
ft.update(b, v, v.Args[0], signed, r)
}
case OpSub64, OpSub32, OpSub16, OpSub8:
x := ft.limits[v.Args[0].ID]
y := ft.limits[v.Args[1].ID]
if !unsignedSubUnderflows(x.umin, y.umax) {
r := lt
if !y.nonzero() {
r |= eq
}
ft.update(b, v, v.Args[0], unsigned, r)
}
// FIXME: we could also do signed facts but the overflow checks are much trickier and I don't need it yet.
case OpAnd64, OpAnd32, OpAnd16, OpAnd8:
ft.update(b, v, v.Args[0], unsigned, lt|eq)
ft.update(b, v, v.Args[1], unsigned, lt|eq)