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4a6f656c opened this issue Apr 30, 2015 · 10 comments
Closed

runtime: math.Sqrt mishandled when GOARM=5 #10641

4a6f656c opened this issue Apr 30, 2015 · 10 comments
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@4a6f656c
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Currently GOARM=5 platforms (on at least the linux-arm-arm5 and openbsd-arm builders) are failing the test/bench/shootout/nbody test due to invalid floating point results - this appears to be caused by a compiler or softfloat related bug. The failure is reproducible with the following:

package main

import (
        "fmt"
        "math"
)

func main() {
        dt := 0.1

        dx := -4.841431442464721
        dy := 1.1603200440274284
        dz := 0.10362204447112311

        dSquared := dx*dx + dy*dy + dz*dz
        distance := math.Sqrt(dSquared)
        mag := dt / (dSquared * distance)

        fmt.Println(mag)
}

Adding a fmt.Println(dSquared) immediately after dSquared is assigned "fixes" the problem and produces correct results.

The main from go tool objdump of the broken version contains:

TEXT main.main(SB) /tmp/nbody.go
        nbody.go:8      0x10c00 e59a1008        MOVW 0x8(R10), R1
        nbody.go:8      0x10c04 e15d0001        CMP R1, R13
        nbody.go:8      0x10c08 91a0300e        MOVW.LS R14, R3
        nbody.go:8      0x10c0c 9b0151bd        BL.LS runtime.morestack_noctxt(SB)
        nbody.go:8      0x10c10 9afffffa        B.LS main.main(SB)
        nbody.go:8      0x10c14 e52de040        MOVW.W R14, -0x40(R13)
        nbody.go:9      0x10c18 eb015ead        BL _sfloat(SB)
        nbody.go:9      0x10c1c e59fb0e4        MOVW 0xe4(R15), R11
        nbody.go:9      0x10c20 ed9b5b00        VLDR [R11], D5
        nbody.go:11     0x10c24 e59fb0e0        MOVW 0xe0(R15), R11
        nbody.go:11     0x10c28 ed9b2b00        VLDR [R11], D2
        nbody.go:12     0x10c2c e59fb0dc        MOVW 0xdc(R15), R11
        nbody.go:12     0x10c30 ed9b4b00        VLDR [R11], D4
        nbody.go:13     0x10c34 e59fb0d8        MOVW 0xd8(R15), R11
        nbody.go:13     0x10c38 ed9b3b00        VLDR [R11], D3
        nbody.go:15     0x10c3c ee220b02        VMUL.F64 D2, D2, D0
        nbody.go:15     0x10c40 ee241b04        VMUL.F64 D4, D4, D1
        nbody.go:15     0x10c44 ee300b01        VADD.F64 D1, D0, D0
        nbody.go:15     0x10c48 ee231b03        VMUL.F64 D3, D3, D1
        nbody.go:15     0x10c4c ee300b01        VADD.F64 D1, D0, D0
        nbody.go:15     0x10c50 eeb02b40        VMOV.F64 D0, D2
        nbody.go:16     0x10c54 eeb10bc0        VSQRT.F64 D0, D0
        nbody.go:16     0x10c58 eeb03b40        VMOV.F64 D0, D3
        nbody.go:17     0x10c5c eb015e9c        BL _sfloat(SB)
        nbody.go:17     0x10c60 ee220b00        VMUL.F64 D0, D2, D0
        nbody.go:17     0x10c64 ee851b00        VDIV.F64 D0, D5, D1
        nbody.go:17     0x10c68 eeb02b41        VMOV.F64 D1, D2
        nbody.go:19     0x10c6c ed8d1b07        VSTR [SP, #28], D1
        nbody.go:19     0x10c70 e3a00000        MOVW $0, R0
        nbody.go:19     0x10c74 e58d002c        MOVW R0, 0x2c(R13)
        nbody.go:19     0x10c78 e58d0030        MOVW R0, 0x30(R13)
        nbody.go:19     0x10c7c e28d202c        ADD $44, R13, R2
        nbody.go:19     0x10c80 e3520000        CMP $0, R2
        nbody.go:19     0x10c84 05822000        MOVW.EQ R2, (R2)
        nbody.go:19     0x10c88 e3a04001        MOVW $1, R4
        nbody.go:19     0x10c8c e3a03001        MOVW $1, R3
        nbody.go:19     0x10c90 e58d2034        MOVW R2, 0x34(R13)
        nbody.go:19     0x10c94 e58d4038        MOVW R4, 0x38(R13)
        nbody.go:19     0x10c98 e58d303c        MOVW R3, 0x3c(R13)
        nbody.go:19     0x10c9c e59f0074        MOVW 0x74(R15), R0
        nbody.go:19     0x10ca0 e58d0004        MOVW R0, 0x4(R13)
        nbody.go:19     0x10ca4 e28d001c        ADD $28, R13, R0
        nbody.go:19     0x10ca8 e58d0008        MOVW R0, 0x8(R13)
        nbody.go:19     0x10cac e3a00000        MOVW $0, R0
        nbody.go:19     0x10cb0 e58d000c        MOVW R0, 0xc(R13)
        nbody.go:19     0x10cb4 eb002812        BL runtime.convT2E(SB)
        nbody.go:19     0x10cb8 e59d3010        MOVW 0x10(R13), R3
        nbody.go:19     0x10cbc e59d2014        MOVW 0x14(R13), R2
        nbody.go:19     0x10cc0 e28d0034        ADD $52, R13, R0
        nbody.go:19     0x10cc4 e5900000        MOVW (R0), R0
        nbody.go:19     0x10cc8 e2800000        ADD $0, R0, R0
        nbody.go:19     0x10ccc e58d0004        MOVW R0, 0x4(R13)
        nbody.go:19     0x10cd0 e58d3024        MOVW R3, 0x24(R13)
        nbody.go:19     0x10cd4 e58d3008        MOVW R3, 0x8(R13)
        nbody.go:19     0x10cd8 e58d2028        MOVW R2, 0x28(R13)
        nbody.go:19     0x10cdc e58d200c        MOVW R2, 0xc(R13)
        nbody.go:19     0x10ce0 eb0035d5        BL runtime.writebarrieriface(SB)
        nbody.go:19     0x10ce4 e59d0034        MOVW 0x34(R13), R0
        nbody.go:19     0x10ce8 e58d0004        MOVW R0, 0x4(R13)
        nbody.go:19     0x10cec e59d0038        MOVW 0x38(R13), R0
        nbody.go:19     0x10cf0 e58d0008        MOVW R0, 0x8(R13)
        nbody.go:19     0x10cf4 e59d003c        MOVW 0x3c(R13), R0
        nbody.go:19     0x10cf8 e58d000c        MOVW R0, 0xc(R13)
        nbody.go:19     0x10cfc eb017223        BL fmt.Println(SB)
        nbody.go:20     0x10d00 e49df040        RET #64
        nbody.go:20     0x10d04 eafffffe        B 0x10d04
        nbody.go:20     0x10d08 0011e368        AND.S.EQ R8@>$6, R1, R14
        nbody.go:20     0x10d0c 0011e3e8        AND.S.EQ R8@>$7, R1, R14
        nbody.go:20     0x10d10 0011e3a0        AND.S.EQ R0>>$7, R1, R14
        nbody.go:20     0x10d14 0011e370        AND.S.EQ R0@>R3, R1, R14
        nbody.go:20     0x10d18 000cec78        AND.EQ R8@>R12, R12, R14

While the working version with the additional fmt.Println has:

TEXT main.main(SB) /tmp/nbody.go
        nbody.go:8      0x10c00 e59a1008        MOVW 0x8(R10), R1
        nbody.go:8      0x10c04 e15d0001        CMP R1, R13
        nbody.go:8      0x10c08 91a0300e        MOVW.LS R14, R3
        nbody.go:8      0x10c0c 9b0151e6        BL.LS runtime.morestack_noctxt(SB)
        nbody.go:8      0x10c10 9afffffa        B.LS main.main(SB)
        nbody.go:8      0x10c14 e52de05c        MOVW.W R14, -0x5c(R13)
        nbody.go:9      0x10c18 eb015ed6        BL _sfloat(SB)
        nbody.go:9      0x10c1c e59fb188        MOVW 0x188(R15), R11
        nbody.go:9      0x10c20 ed9b0b00        VLDR [R11], D0
        nbody.go:9      0x10c24 ed8d0b07        VSTR [SP, #28], D0
        nbody.go:11     0x10c28 e59fb180        MOVW 0x180(R15), R11
        nbody.go:11     0x10c2c ed9b2b00        VLDR [R11], D2
        nbody.go:12     0x10c30 e59fb17c        MOVW 0x17c(R15), R11
        nbody.go:12     0x10c34 ed9b4b00        VLDR [R11], D4
        nbody.go:13     0x10c38 e59fb178        MOVW 0x178(R15), R11
        nbody.go:13     0x10c3c ed9b3b00        VLDR [R11], D3
        nbody.go:15     0x10c40 ee220b02        VMUL.F64 D2, D2, D0
        nbody.go:15     0x10c44 ee241b04        VMUL.F64 D4, D4, D1
        nbody.go:15     0x10c48 ee300b01        VADD.F64 D1, D0, D0
        nbody.go:15     0x10c4c ee231b03        VMUL.F64 D3, D3, D1
        nbody.go:15     0x10c50 ee300b01        VADD.F64 D1, D0, D0
        nbody.go:15     0x10c54 eeb02b40        VMOV.F64 D0, D2
        nbody.go:16     0x10c58 ed8d0b09        VSTR [SP, #36], D0
        nbody.go:16     0x10c5c ed8d0b0b        VSTR [SP, #44], D0
        nbody.go:16     0x10c60 e3a00000        MOVW $0, R0
        nbody.go:16     0x10c64 e58d003c        MOVW R0, 0x3c(R13)
        nbody.go:16     0x10c68 e58d0040        MOVW R0, 0x40(R13)
        nbody.go:16     0x10c6c e28d203c        ADD $60, R13, R2
        nbody.go:16     0x10c70 e3520000        CMP $0, R2
        nbody.go:16     0x10c74 05822000        MOVW.EQ R2, (R2)
        nbody.go:16     0x10c78 e3a04001        MOVW $1, R4
        nbody.go:16     0x10c7c e3a03001        MOVW $1, R3
        nbody.go:16     0x10c80 e58d2050        MOVW R2, 0x50(R13)
        nbody.go:16     0x10c84 e58d4054        MOVW R4, 0x54(R13)
        nbody.go:16     0x10c88 e58d3058        MOVW R3, 0x58(R13)
        nbody.go:16     0x10c8c e59f0128        MOVW 0x128(R15), R0
        nbody.go:16     0x10c90 e58d0004        MOVW R0, 0x4(R13)
        nbody.go:16     0x10c94 e28d002c        ADD $44, R13, R0
        nbody.go:16     0x10c98 e58d0008        MOVW R0, 0x8(R13)
        nbody.go:16     0x10c9c e3a00000        MOVW $0, R0
        nbody.go:16     0x10ca0 e58d000c        MOVW R0, 0xc(R13)
        nbody.go:16     0x10ca4 eb00283f        BL runtime.convT2E(SB)
        nbody.go:16     0x10ca8 e59d3010        MOVW 0x10(R13), R3
        nbody.go:16     0x10cac e59d2014        MOVW 0x14(R13), R2
        nbody.go:16     0x10cb0 e28d0050        ADD $80, R13, R0
        nbody.go:16     0x10cb4 e5900000        MOVW (R0), R0
        nbody.go:16     0x10cb8 e2800000        ADD $0, R0, R0
        nbody.go:16     0x10cbc e58d0004        MOVW R0, 0x4(R13)
        nbody.go:16     0x10cc0 e58d3034        MOVW R3, 0x34(R13)
        nbody.go:16     0x10cc4 e58d3008        MOVW R3, 0x8(R13)
        nbody.go:16     0x10cc8 e58d2038        MOVW R2, 0x38(R13)
        nbody.go:16     0x10ccc e58d200c        MOVW R2, 0xc(R13)
        nbody.go:16     0x10cd0 eb003602        BL runtime.writebarrieriface(SB)
        nbody.go:16     0x10cd4 e59d0050        MOVW 0x50(R13), R0
        nbody.go:16     0x10cd8 e58d0004        MOVW R0, 0x4(R13)
        nbody.go:16     0x10cdc e59d0054        MOVW 0x54(R13), R0
        nbody.go:16     0x10ce0 e58d0008        MOVW R0, 0x8(R13)
        nbody.go:16     0x10ce4 e59d0058        MOVW 0x58(R13), R0
        nbody.go:16     0x10ce8 e58d000c        MOVW R0, 0xc(R13)
        nbody.go:16     0x10cec eb017250        BL fmt.Println(SB)
        nbody.go:16     0x10cf0 eb015ea0        BL _sfloat(SB)
        nbody.go:16     0x10cf4 ed9d3b09        VLDR [SP, #36], D3
        nbody.go:18     0x10cf8 eeb10bc3        VSQRT.F64 D3, D0
        nbody.go:19     0x10cfc eb015e9d        BL _sfloat(SB)
        nbody.go:19     0x10d00 ee230b00        VMUL.F64 D0, D3, D0
        nbody.go:19     0x10d04 ed9d2b07        VLDR [SP, #28], D2
        nbody.go:19     0x10d08 ee822b00        VDIV.F64 D0, D2, D2
        nbody.go:19     0x10d0c eeb01b42        VMOV.F64 D2, D1
        nbody.go:21     0x10d10 ed8d2b0b        VSTR [SP, #44], D2
        nbody.go:21     0x10d14 e3a00000        MOVW $0, R0
        nbody.go:21     0x10d18 e58d003c        MOVW R0, 0x3c(R13)
        nbody.go:21     0x10d1c e58d0040        MOVW R0, 0x40(R13)
        nbody.go:21     0x10d20 e28d203c        ADD $60, R13, R2
        nbody.go:21     0x10d24 e3520000        CMP $0, R2
        nbody.go:21     0x10d28 05822000        MOVW.EQ R2, (R2)
        nbody.go:21     0x10d2c e3a03001        MOVW $1, R3
        nbody.go:21     0x10d30 e3a04001        MOVW $1, R4
        nbody.go:21     0x10d34 e58d2044        MOVW R2, 0x44(R13)
        nbody.go:21     0x10d38 e58d3048        MOVW R3, 0x48(R13)
        nbody.go:21     0x10d3c e58d404c        MOVW R4, 0x4c(R13)
        nbody.go:21     0x10d40 e59f0074        MOVW 0x74(R15), R0
        nbody.go:21     0x10d44 e58d0004        MOVW R0, 0x4(R13)
        nbody.go:21     0x10d48 e28d002c        ADD $44, R13, R0
        nbody.go:21     0x10d4c e58d0008        MOVW R0, 0x8(R13)
        nbody.go:21     0x10d50 e3a00000        MOVW $0, R0
        nbody.go:21     0x10d54 e58d000c        MOVW R0, 0xc(R13)
        nbody.go:21     0x10d58 eb002812        BL runtime.convT2E(SB)
        nbody.go:21     0x10d5c e59d3010        MOVW 0x10(R13), R3
        nbody.go:21     0x10d60 e59d2014        MOVW 0x14(R13), R2
        nbody.go:21     0x10d64 e28d0044        ADD $68, R13, R0
        nbody.go:21     0x10d68 e5900000        MOVW (R0), R0
        nbody.go:21     0x10d6c e2800000        ADD $0, R0, R0
        nbody.go:21     0x10d70 e58d0004        MOVW R0, 0x4(R13)
        nbody.go:21     0x10d74 e58d3034        MOVW R3, 0x34(R13)
        nbody.go:21     0x10d78 e58d3008        MOVW R3, 0x8(R13)
        nbody.go:21     0x10d7c e58d2038        MOVW R2, 0x38(R13)
        nbody.go:21     0x10d80 e58d200c        MOVW R2, 0xc(R13)
        nbody.go:21     0x10d84 eb0035d5        BL runtime.writebarrieriface(SB)
        nbody.go:21     0x10d88 e59d0044        MOVW 0x44(R13), R0
        nbody.go:21     0x10d8c e58d0004        MOVW R0, 0x4(R13)
        nbody.go:21     0x10d90 e59d0048        MOVW 0x48(R13), R0
        nbody.go:21     0x10d94 e58d0008        MOVW R0, 0x8(R13)
        nbody.go:21     0x10d98 e59d004c        MOVW 0x4c(R13), R0
        nbody.go:21     0x10d9c e58d000c        MOVW R0, 0xc(R13)
        nbody.go:21     0x10da0 eb017223        BL fmt.Println(SB)
        nbody.go:22     0x10da4 e49df05c        RET #92
        nbody.go:22     0x10da8 eafffffe        B 0x10da8
        nbody.go:22     0x10dac 0011e368        AND.S.EQ R8@>$6, R1, R14
        nbody.go:22     0x10db0 0011e3e8        AND.S.EQ R8@>$7, R1, R14
        nbody.go:22     0x10db4 0011e3a0        AND.S.EQ R0>>$7, R1, R14
        nbody.go:22     0x10db8 0011e370        AND.S.EQ R0@>R3, R1, R14
        nbody.go:22     0x10dbc 000cec78        AND.EQ R8@>R12, R12, R14
@josharian
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Dup of #10629?

@josharian josharian mentioned this issue Apr 30, 2015
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@4a6f656c
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This was caused by https://go-review.googlesource.com/8356, which failed on the linux-arm-arm5 builder (whereas the previous build completed successfully).

@4a6f656c
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@josharian it would seem so (I had searched with "softfloat" instead of "soft float" - seems github is not smart enough to find it :) - I'm reluctant to close this in favour of the other issue since there is a reproducible test case and assembly dumps here...

@josharian
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Closed #10629; we'll leave this one.

@josharian josharian added this to the Go1.5 milestone Apr 30, 2015
@josharian josharian changed the title arm compiler/softfloat bug (GOARM=5) cmd/5g: math.Sqrt mishandled when GOARM=5 Apr 30, 2015
@davecheney
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Thanks for the much more through issue report.

On Fri, 1 May 2015 02:27 Josh Bleecher Snyder notifications@github.com
wrote:

Closed #10629 #10629; we'll leave
this one.

—
Reply to this email directly or view it on GitHub
#10641 (comment).

@4a6f656c
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I've just sent https://go-review.googlesource.com/9524 to disable OSQRT on GOARM=5, which avoids producing incorrect results until the actual problem can be found/fixed.

@gopherbot
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CL https://golang.org/cl/9524 mentions this issue.

@minux
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minux commented Apr 30, 2015 via email

@minux minux changed the title cmd/5g: math.Sqrt mishandled when GOARM=5 runtime: math.Sqrt mishandled when GOARM=5 Apr 30, 2015
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CL https://golang.org/cl/9570 mentions this issue.

mk0x9 pushed a commit to mk0x9/go that referenced this issue May 5, 2015
cmd/internal: disable OSQRT on GOARM=5
3120adc 
OSQRT currently produces incorrect results when used on arm with softfloat.
Disable it on GOARM=5 until the actual problem is found and fixed.

Updates golang#10641

Change-Id: Ia6f6879fbbb05cb24399c2feee93c1be21113e73
Reviewed-on: https://go-review.googlesource.com/9524
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
@minux minux closed this as completed in 102436e May 5, 2015
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CL https://golang.org/cl/9720 mentions this issue.

minux added a commit that referenced this issue May 5, 2015
@minux
Revert "cmd/internal: disable OSQRT on GOARM=5"
8fb55f9 
I just submitted the real fix for #10641.

This reverts commit 3120adc.

Change-Id: I55051515f697e27ca887ed21c2ac985f0b9b062b
Reviewed-on: https://go-review.googlesource.com/9720
Reviewed-by: Joel Sing <jsing@google.com>
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