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commit 6d9ee1389572ae985f6a39bb99dbd10cdf42c123
parent e0b94a3d6ade2f99ded481318e9e6d9f16953662
Author: Michael Forney <mforney@mforney.org>
Date:   Wed, 16 Jun 2021 20:27:49 -0700

amd64: fix conditional jump when compare is swapped and used elsewhere

selcmp may potentially swap the arguments and return 1 indicating
that the opposite operation should be used. However, if the compare
result is used for a conditional jump as well as elsewhere, the
original compare op is used instead of the opposite.

To fix this, add a check to see whether the opposite compare should
be used, regardless of whether selcmp() is done now, or later on
during sel().

Bug report and test case from Charlie Stanton.

Diffstat:
Mamd64/isel.c | 5+++--
Atest/cmp1.ssa | 17+++++++++++++++++
2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/amd64/isel.c b/amd64/isel.c @@ -383,9 +383,10 @@ seljmp(Blk *b, Fn *fn) b->jmp.type = Jjf + Cine; } else if (iscmp(fi->op, &k, &c)) { + if (rtype(fi->arg[0]) == RCon) + c = cmpop(c); if (t->nuse == 1) { - if (selcmp(fi->arg, k, fn)) - c = cmpop(c); + selcmp(fi->arg, k, fn); *fi = (Ins){.op = Onop}; } b->jmp.type = Jjf + c; diff --git a/test/cmp1.ssa b/test/cmp1.ssa @@ -0,0 +1,17 @@ +# test cmp used in jnz as well as its result value + +export +function w $test(w %c) { +@start + %cmp =w cultw 1, %c + jnz %cmp, @yes, @no +@yes + %cmp =w copy 1 +@no + ret %cmp +} + +# >>> driver +# int test(int); +# int main(void) { return test(0); } +# <<<