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cmd/compile: miscompilation of some arithmetic and conditionals on arm [1.15 backport] #42930

gopherbot opened this issue Dec 1, 2020 · 3 comments
CherryPickApproved FrozenDueToAge


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@gopherbot gopherbot commented Dec 1, 2020

@zx2c4 requested issue #42876 to be considered for backport to the next 1.15 minor release.

@gopherbot please backport this because compiler bugs in bitmath seem worrisome

@gopherbot gopherbot added the CherryPickCandidate label Dec 1, 2020
@gopherbot gopherbot added this to the Go1.15.6 milestone Dec 1, 2020
@cagedmantis cagedmantis removed this from the Go1.15.6 milestone Dec 3, 2020
@cagedmantis cagedmantis added this to the Go1.15.7 milestone Dec 3, 2020
@toothrot toothrot added the CherryPickApproved label Jan 7, 2021
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@toothrot toothrot commented Jan 7, 2021

Approving as a serious issue with no workaround.

@gopherbot gopherbot removed the CherryPickCandidate label Jan 7, 2021
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@gopherbot gopherbot commented Jan 7, 2021

Change mentions this issue: [release-branch.go1.15] cmd/compile: do not assume TST and TEQ set V on arm

@dmitshur dmitshur removed this from the Go1.15.7 milestone Jan 19, 2021
@dmitshur dmitshur added this to the Go1.15.8 milestone Jan 19, 2021
@cagedmantis cagedmantis removed this from the Go1.15.8 milestone Feb 4, 2021
@cagedmantis cagedmantis added this to the Go1.15.9 milestone Feb 4, 2021
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@gopherbot gopherbot commented Mar 1, 2021

Closed by merging d9b4ef5 to release-branch.go1.15.

gopherbot pushed a commit that referenced this issue Mar 1, 2021
…on arm

These replacement rules assume that TST and TEQ set V. But TST and
TEQ do not set V. This is a problem because instructions like LT are
actually checking for N!=V. But with TST and TEQ not setting V, LT
doesn't do anything meaningful. It's possible to construct trivial
miscompilations from this, such as:

    package main

    var x = [4]int32{-0x7fffffff, 0x7fffffff, 2, 4}

    func main() {
        if x[0] > x[1] {
            panic("fail 1")
        if x[2]&x[3] < 0 {
            panic("fail 2") // Fails here

That first comparison sets V, via the CMP that subtracts the values
causing the overflow. Then the second comparison operation thinks that
it uses the result of TST, when it actually uses the V from CMP.

Before this fix:

    TST             R0, R1
    BLT             loc_6C164

After this fix:

    TST             R0, R1
    BMI             loc_6C164

The BMI instruction checks the N flag, which TST sets.  This commit
fixes the issue by using [LG][TE]noov instead of vanilla [LG][TE], and
also adds a test case for the direct issue.

Updates #42876.
Fixes #42930.

Change-Id: I13c62c88d18574247ad002b671b38d2d0b0fc6fa
Trust: Jason A. Donenfeld <>
Run-TryBot: Jason A. Donenfeld <>
TryBot-Result: Go Bot <>
Reviewed-by: Cherry Zhang <>
@toothrot toothrot removed this from the Go1.15.9 milestone Mar 10, 2021
@toothrot toothrot added this to the Go1.15.10 milestone Mar 10, 2021
dweomer added a commit to dweomer/k3s that referenced this issue Mar 19, 2021
See golang/go#42930 which addresses an issue
on arm.

Signed-off-by: Jacob Blain Christen <>
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