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a46ecdca36
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. Fixes #66066 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>
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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