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cmd/compile: SSA, use CMPconst for write barrier #14587

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randall77 opened this issue Mar 1, 2016 · 2 comments
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cmd/compile: SSA, use CMPconst for write barrier #14587

randall77 opened this issue Mar 1, 2016 · 2 comments
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@randall77
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right now we generate this code for a write barrier test:

MOVL    runtime.writeBarrier(SB), AX
TESTB   AL, AL
JNE ...

We should really do:

CMPB    $1, runtime.writeBarrier(SB)
JEQ ...

One less instruction, no register required.
It's not easy to do now because flagalloc doesn't like operations that take a memory input and generate a flag output (because it wants to be able to move the flag-generation instructions around).

Figure out a way to fix this. Maybe introduce a new block type BlockWBCheck that takes a memory control value and has 2 successors. It has the CMPx implicit inside it. It would need to be marked as clobbering flags.

@drchase
@minux
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minux commented Mar 1, 2016

If the only use of a load is a CMP with const,
I think we could combine the load and CMP
into CMP $const, MEM.

Actually, this transform should be generic
for x86 and not necessarily limited to CMP.

I think having a package global bool that
is queried often is a pretty common scenario.
(it probably also makes sense to align such
variable to cache line size, so that there is
no false sharing.)

@minux minux mentioned this issue Mar 31, 2016
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@bradfitz bradfitz modified the milestone: Unplanned Apr 9, 2016
@randall77
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This is done.

@golang golang locked and limited conversation to collaborators Apr 1, 2020
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