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cmd/compile: stack offsets should not need spilling #22947

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mdempsky opened this issue Nov 30, 2017 · 2 comments

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commented Nov 30, 2017

Compiling the function below currently produces a register spill:

package p

type T [100]int

func f(p *T) {
    if g() {
            h(0, *p)
    } else {
            g()
            h(1, *p)
    }
}

func g() bool
func h(int, T)

This is because h(1, *p) is compiled to OINDREGSP(8) = *p, which is represented in SSA as Move(OffPtr(8, SP), p).

OffPtr(8, SP) is trivially rematerialized on x86 with LEA $8(SP), but we're currently lowering OffPtr to ADDQconst, which is marked as non-rematerializeable because it clobbers flags (unlike LEAQ).

Distilled from #22558.

/cc @randall77

@mdempsky mdempsky added this to the Go1.11 milestone Nov 30, 2017

@TocarIP

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commented Dec 1, 2017

We can probably add a rule that transforms (ADDQconst (SP)) into LEAQ

@gopherbot

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commented Dec 4, 2017

Change https://golang.org/cl/81815 mentions this issue: cmd/compile/internal/ssa: don't spill register offsets on amd64

@gopherbot gopherbot closed this in 9916fee Feb 14, 2018

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