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cmd/compile: extra "xchgl al, al" instructions #38255

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egonelbre opened this issue Apr 5, 2020 · 2 comments
Closed

cmd/compile: extra "xchgl al, al" instructions #38255

egonelbre opened this issue Apr 5, 2020 · 2 comments

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@egonelbre
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@egonelbre egonelbre commented Apr 5, 2020

Noticed a case where compiler produces a repeated sequence of XCHGL AX, AX.

https://go.godbolt.org/z/_yJaga

What version of Go are you using (go version)?

$ go version
go version go1.14.1 windows/amd64

But also happens on

  • go1.13.9 and
  • go version devel +e31d741801 Fri Apr 3 17:51:12 2020 +0000 windows/amd64

What did you see?

Compiling https://play.golang.org/p/HfxglRpLB4A with go tool compile -S shows that the output contains a sequence:

        0x008c 00140 ($GOROOT\src\io\io.go:329) CALL    io.ReadAtLeast(SB)
        0x0091 00145 ($GOROOT\src\io\io.go:329) CMPQ    56(SP), $0
        0x0097 00151 (main.go:10)       JEQ     169
        0x0099 00153 (main.go:11)       PCDATA  $0, $-1
        0x0099 00153 (main.go:11)       PCDATA  $1, $-1
        0x0099 00153 (main.go:11)       MOVQ    144(SP), BP
        0x00a1 00161 (main.go:11)       ADDQ    $152, SP
        0x00a8 00168 (main.go:11)       RET
        0x00a9 00169 (main.go:16)       XCHGL   AX, AX
        0x00aa 00170 (main.go:19)       XCHGL   AX, AX
        0x00ab 00171 (main.go:22)       XCHGL   AX, AX
        0x00ac 00172 (main.go:25)       XCHGL   AX, AX
        0x00ad 00173 (main.go:28)       XCHGL   AX, AX
        0x00ae 00174 (main.go:31)       XCHGL   AX, AX
        0x00af 00175 (main.go:34)       XCHGL   AX, AX
        0x00b0 00176 (main.go:37)       XCHGL   AX, AX
        0x00b1 00177 ($GOROOT\src\encoding\binary\binary.go:78) PCDATA  $0, $1
        0x00b1 00177 ($GOROOT\src\encoding\binary\binary.go:78) PCDATA  $1, $3
        0x00b1 00177 ($GOROOT\src\encoding\binary\binary.go:78) MOVQ    ""..autotmp_80+136(SP), AX

I would expect that a single XCHGL AX, AX is sufficient.

@egonelbre
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@egonelbre egonelbre commented Apr 5, 2020

It seems these are generated due to InlMark and are left together after "lowered deadcode for cse" pass.

Possibly related #29571

@randall77
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@randall77 randall77 commented Apr 5, 2020

Those are just the inline marks. We need so many because each represents a different inlining at a different source line.

Yes, this is a dup of #29571.

There are things we could do here, like put the inline marks out-of-line. But let's discuss that over at #29571.

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