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cmd/compile: stores in infinite loops disappear #15635

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randall77 opened this issue May 10, 2016 · 4 comments
Closed

cmd/compile: stores in infinite loops disappear #15635

randall77 opened this issue May 10, 2016 · 4 comments
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@randall77
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func f(p *int) {
    _ = *p // nil check
    for i := 0; true; i++ {
        *p = i
    }
}

With SSA, this compiles to:

    0x0000 00000 (infloop.go:4) MOVQ    "".p+8(FP), AX
    0x0005 00005 (infloop.go:4) TESTB   AL, (AX)
    0x0007 00007 (infloop.go:5) JMP 7

There are no reads of memory in the loop, so the entire store chain gets optimized away.

Is this a problem? I'm not sure. It is unexpected (at least, to me) so it may be worth fixing. Opinions? @rsc @josharian @griesemer @mdempsky @dr2chase @ianlancetaylor

This issue may prevent the optimization described in #15631 (treating calls just like stores). We definitely don't want calls vanishing.
@griesemer
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It's only observable if we have another goroutine watching that variable (*p). But for another goroutine to see the actual variable, Go requires some communication between the goroutines (sync or channel send). So it might be permissible to do nothing.

@randall77
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FWIW, gcc -O1 and above also drop the writes from the equivalent C code.

@mdempsky
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Agreed with @griesemer that the Go spec + memory model allow this optimization. The loop never terminates and doesn't have any synchronization events, so there's no way for another goroutine to guarantee any of the *p = i assignments have happened.

@randall77
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Ok, sounds like this is allowed. I'll close.

@golang golang locked and limited conversation to collaborators May 13, 2017
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@mdempsky @randall77 @griesemer @gopherbot