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> |
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