cmd/compile: extra unused instructions for LoweredNilCheck #17211
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MOVB is an AMD64-ism -- PPC has no such instruction, so the assembler inserts the (unnecessary) sign extension. |
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But why is the load instruction even there when it is loading it into a scratch register that is never used anywhere? If a NilCheck is supposed to be checking for a nil pointer, can't it just do a compare. |
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Certainly on AMD64, it's fastest to do a useless load. A branch would require a place to branch to, etc. Is there a better way for PPC? |
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The point for this NilCheck is that if it is a nil pointer, the load will trap. You are right that the extension is unnecessary. Using MOVD would not work as it will also trap on pointer that is not 8-aligned. |
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CL https://golang.org/cl/29762 mentions this issue. |
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Please answer these questions before submitting your issue. Thanks!
What version of Go are you using (
go version)?go version devel +dcbbd31 Thu Sep 22 06:41:11 2016 +0000 linux/ppc64le
What operating system and processor architecture are you using (
go env)?Ubuntu 16.04 ppc64le
Looking at generated code for code improvements.
This appears in many procedures and is never used (example found in runtime.c64hash):
Due to:
v26 = LoadReg <unsafe.Pointer> v8 : R3
v10 = LoweredNilCheck v26 v1
asm:
MOVB (R3), R31
With generated code:
11534: 00 00 e3 8b lbz r31,0(r3)
11538: 74 07 ff 7f extsb r31,r31
I'm not sure what it's purpose is. To avoid the extsb I believe I can just change this to a MOVD instead of a MOVB for the LoweredNilCheck, but r31 is never used in this function and the pointer for r3 is never dereferenced.
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