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46587483e3
When an opcode generates a known high bit state (typically, a sub-word
operation that zeros the high bits), we can remove any subsequent
extension operation that would be a no-op.
x = (OP ...)
y = (ZeroExt32to64 x)
If OP zeros the high 32 bits, then we can replace y with x, as the
zero extension doesn't do anything.
However, x in this situation normally has a sub-word-sized type. The
semantics of values in registers is typically that the high bits
beyond the value's type size are junk. So although the opcode
generating x *currently* zeros the high bits, after x is rewritten to
another opcode it may not - rewrites of sub-word-sized values can
trash the high bits.
To fix, move the extension-removing rules to late lower. That ensures
that their arguments won't be rewritten to change their high bits.
I am also worried about spilling and restoring. Spilling and restoring
doesn't preserve the high bits, but instead sets them to a known value
(often 0, but in some cases it could be sign-extended). I am unable
to come up with a case that would cause a problem here, so leaving for
another time.
Update #66076
Change-Id: I3b5c091b3b3278ccbb7f11beda8b56f4b6d3fde7
Reviewed-on: https://go-review.googlesource.com/c/go/+/568616
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
(cherry picked from commit a46ecdca36)
Reviewed-on: https://go-review.googlesource.com/c/go/+/573375
42 lines
677 B
Go
42 lines
677 B
Go
// run
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// Copyright 2024 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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package main
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import "fmt"
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func main() {
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testMod()
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testMul()
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}
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//go:noinline
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func mod3(x uint32) uint64 {
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return uint64(x % 3)
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}
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func testMod() {
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got := mod3(1<<32 - 1)
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want := uint64((1<<32 - 1) % 3)
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if got != want {
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fmt.Printf("testMod: got %x want %x\n", got, want)
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}
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}
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//go:noinline
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func mul3(a uint32) uint64 {
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return uint64(a * 3)
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}
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func testMul() {
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got := mul3(1<<32 - 1)
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want := uint64((1<<32-1)*3 - 2<<32)
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if got != want {
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fmt.Printf("testMul: got %x want %x\n", got, want)
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}
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}
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