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cmd/compile: GOARM=5 bad floating point #15440

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randall77 opened this issue Apr 25, 2016 · 3 comments
Closed

cmd/compile: GOARM=5 bad floating point #15440

randall77 opened this issue Apr 25, 2016 · 3 comments
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Go1.7

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@randall77
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Looks like the floating-point simulator for arm5 can't handle floating point writes at large offsets to pointers.

func TestArmFloatBigOffset(t *testing.T) {
    type X struct {
        pad [1024]byte
        v   float64
    }
    f := func(x *X, v float64) {
        x.v = v
    }
    var x X
    f(&x, 2.0)
    if v := x.v; v != 2.0 {
        t.Errorf("bad x.v=%f\n", v)
    }
}

The Go compiler compiles the write in f to:

    0x0014 00020 (tmp2.go:9)    CALL    _sfloat(SB)
    0x0018 00024 (tmp2.go:9)    MOVD    "".v+4(FP), F0
    0x001c 00028 (tmp2.go:9)    MOVD    F0, 1024(R0)

which all looks ok, until you see what the obj library did to it:

   11014:       eb017942        bl      6f524 <_sfloat>
   11018:       ed9d0b03        vldr    d0, [sp, #12]
   1101c:       e59fb018        ldr     fp, [pc, #24]   ; 1103c <main.f+0x3c>
   11020:       e08bb000        add     fp, fp, r0
   11024:       ed8b0b00        vstr    d0, [fp]
   ...
   1103c:       00000400        andeq   r0, r0, r0, lsl #8

The problem (I think) is the sfloat interpreter stops interpreting when it gets to the ldr, and the store doesn't work due to there not being a sfloat call before it.

This issue is what is currently breaking the linux-arm-arm5 build.

@minux
@randall77 randall77 added this to the Go1.7 milestone Apr 25, 2016
@mwhudson
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I actually think it's

   11020:       e08bb000        add     fp, fp, r0

that's not being interpreted. Should be easy enough though.

@randall77
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Yes, looks pretty easy. Fix for review: https://go-review.googlesource.com/22440

@randall77 randall77 self-assigned this Apr 25, 2016
@gopherbot
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CL https://golang.org/cl/22440 mentions this issue.

@golang golang locked and limited conversation to collaborators Apr 25, 2017
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@mwhudson @randall77 @gopherbot