https://bugs.llvm.org/show_bug.cgi?id=33695
Bug ID: 33695
Summary: InstCombine does invalid transformation
Product: new-bugs
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: babo...@gmail.com
CC: llvm-bugs@lists.llvm.org
clang trunk (also reproducible with 4.0, not reproducible with 3.9), x86_64.
> cat f.cpp
#include <stdio.h>
unsigned char uc = 170;
unsigned char var_1 = 252;
unsigned int var_33 = 1346838541U;
unsigned char var_36;
void foo() {
var_36 = (((var_33 | (0xaf2597a4 | uc)) << 26) - var_1) * 70983U;
}
int main() {
foo();
printf("result: %d\n", var_36);
return 0;
}
> clang++ -O0 f.cpp -o out0
> ./out0
result: 28
> clang++ -O2 f.cpp -o out2
> ./out2
result: 0
Opt-bisect points to "Combine redundant instructions on function", which
removes the calculations and foo() and stores 0 to var_36, which is incorrect
value.
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