8000 cmd/compile: memcombine doesn't combine simple chain · Issue #72832 · golang/go · GitHub
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dominikh opened this issue Mar 13, 2025 · 7 comments
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cmd/compile: memcombine doesn't combine simple chain #72832

dominikh opened this issue Mar 13, 2025 · 7 comments
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BugReport Issues describing a possible bug in the Go implementation. compiler/runtime Issues related to the Go compiler and/or runtime. help wanted NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. Performance
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Unplanned

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@dominikh
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dominikh commented Mar 13, 2025 •
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Go version

go version go1.24.1 linux/amd64

Output of go env in your module/workspace:

-

What did you do?

go build -gcflags=-S

package pkg

var foo1 tile1
var foo2 tile2
var foo3 tile3

type tile1 struct {
	a uint16
	b uint16
	c uint32
}

type tile2 struct {
	a uint16
	b uint16
	c uint16
	d uint16
}

type tile3 struct {
	a uint16
	b uint16
	c uint8
	d uint8
	e uint16
}

func (a *tile1) store() { foo1 = *a }
func (a *tile2) store() { foo2 = *a }
func (a *tile3) store() { foo3 = *a }

What did you see happen?

tile2.store and tile3.store compile to MOVQ, as expected. tile1.store, however, uses two MOVL instead.

command-line-arguments.(*tile1).store STEXT nosplit size=18 args=0x8 locals=0x0 funcid=0x0 align=0x0
        0x0000 00000 (bar.go:28)     MOVL    (AX), CX
        0x0002 00002 (bar.go:28)     MOVL    4(AX), AX
        0x0005 00005 (bar.go:28)     MOVL    CX, command-line-arguments.foo1(SB)
        0x000b 00011 (bar.go:28)     MOVL    AX, command-line-arguments.foo1+4(SB)
        0x0011 00017 (bar.go:28)     RET

command-line-arguments.(*tile2).store STEXT nosplit size=11 args=0x8 locals=0x0 funcid=0x0 align=0x0
        0x0000 00000 (bar.go:29)     MOVQ    (AX), AX
        0x0003 00003 (bar.go:29)     MOVQ    AX, command-line-arguments.foo2(SB)
        0x000a 00010 (bar.go:29)     RET

command-line-arguments.(*tile3).store STEXT nosplit size=11 args=0x8 locals=0x0 funcid=0x0 align=0x0
        0x0000 00000 (bar.go:30)     MOVQ    (AX), AX
        0x0003 00003 (bar.go:30)     MOVQ    AX, command-line-arguments.foo3(SB)
        0x000a 00010 (bar.go:30)     RET

What did you expect to see?

I expected all 3 methods to compile the same, using MOVQ.
@gopherbot gopherbot added the compiler/runtime Issues related to the Go compiler and/or runtime. label Mar 13, 2025
@gabyhelp
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Related Issues

(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)

@gabyhelp gabyhelp added the BugReport Issues describing a possible bug in the Go implementation. label Mar 13, 2025
@dr2chase
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@golang/compiler

@dr2chase dr2chase added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Mar 14, 2025
@randall77
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memcombine is designed to handle patterns generated by encoding/binary operations, it is not (currently) expected to handle everything.

That said, it handles the tile2 case. tile3 is handled by a different mechanism entirely, as it takes the large struct path. That uses a whole-struct move instead of field-by-field moves.

tile1 fails because we process back to front, and at the start we can't find a pair for the uint32. We then later figure out that we can pair the two uint16s into a uint32. But we never go back and try again to pair the first uint32. Maybe a retry when some rewriting happens is all it will take to handle this issue. CLs welcome.

@randall77
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The other option is to implement the TODO described here:

go/src/cmd/compile/internal/ssa/memcombine.go

Line 504 in 580b6ee

// TODO: the constant source and consecutive load source cases

@dominikh
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How come that the mechanism used for tile3 doesn't work for tile1? They're both the same size, after all.

@randall77
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The current rule to move to the large-struct regime is > 4 fields or > 4 words of memory.

@mknyszek mknyszek added this to the Unplanned milestone Mar 19, 2025
@gopherbot
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8000 gopherbot commented Mar 31, 2025

Change https://go.dev/cl/661855 mentions this issue: cmd/compile: run memcombine until converge

@cherrymui cherrymui mentioned this issue May 14, 2025
Open
@github-project-automation github-project-automation bot moved this from Todo to Done in Go Compiler / Runtime May 20, 2025
@prattmic prattmic mentioned this issue May 21, 2025
Closed
Sirherobrine23 pushed a commit to Sirherobrine23/go that referenced this issue May 24, 2025
@JunyangShao @Sirherobrine23
cmd/compile: memcombine different size stores
94b6444 
This CL implements the TODO in combineStores to allow combining
stores of different sizes, as long as the total size aligns to
2, 4, 8.

Fixes golang#72832.

Change-Id: I6d1d471335da90d851ad8f3b5a0cf10bdcfa17c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/661855
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
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BugReport Issues describing a possible bug in the Go implementation. compiler/runtime Issues related to the Go compiler and/or runtime. help wanted NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. Performance
Projects
Go Compiler / Runtime
Status: Done
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@dominikh @mknyszek @dr2chase @randall77 @gopherbot @cherrymui @gabyhelp
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