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9febe9e4be
On top of the previous widening_mul patch, this one recognizes also
(non-perfect) signed multiplication with overflow, like:
int
f5 (int x, int y, int *res)
{
*res = (unsigned) x * y;
return x && (*res / x) != y;
}
The problem with such checks is that they invoke UB if x is -1 and
y is INT_MIN during the division, but perhaps the code knows that
those values won't appear. As that case is UB, we can do for that
case whatever we want and handling that case as signed overflow
is the best option. If x is a constant not equal to -1, then the checks
are 100% correct though.
Haven't tried to pattern match bullet-proof checks, because I really don't
know if users would write it in real-world code like that,
perhaps
*res = (unsigned) x * y;
return x && (x == -1 ? (*res / y) != x : (*res / x) != y);
?
https://wiki.sei.cmu.edu/confluence/display/c/INT32-C.+Ensure+that+operations+on+signed+integers+do+not+result+in+overflow
suggests to use twice as wide multiplication (perhaps we should handle that
too, for both signed and unsigned), or some very large code
with 4 different divisions nested in many conditionals, no way one can
match all the possible variants thereof.
2021-01-11 Jakub Jelinek <jakub@redhat.com>
PR tree-optimization/95852
* tree-ssa-math-opts.c (maybe_optimize_guarding_check): Change
mul_stmts parameter type to vec<gimple *> &. Before cond_stmt
allow in the bb any of the stmts in that vector, div_stmt and
up to 3 cast stmts.
(arith_cast_equal_p): New function.
(arith_overflow_check_p): Add cast_stmt argument, handle signed
multiply overflow checks.
(match_arith_overflow): Adjust caller. Handle signed multiply
overflow checks.
* gcc.target/i386/pr95852-3.c: New test.
* gcc.target/i386/pr95852-4.c: New test.
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