| Issue |
115683
|
| Summary |
Missed optimization: (y != 0 && x > (unsigned)(-1) / y) (multiplication overflow check)
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
Explorer09
|
```c
#include <limits.h>
#include <stdbool.h>
bool func1(unsigned long long x, unsigned long long y)
{
return x > (ULLONG_MAX / y);
}
bool func2(unsigned long long x, unsigned long long y)
{
return y != 0 && x > (ULLONG_MAX / y);
}
bool func3(unsigned long long x, unsigned long long y)
{
unsigned long long res;
return __builtin_umulll_overflow(x, y, &res);
}
```
Expected result: All three functions produce the same code.
Actual result: func1() and func3() optimize to same code, but func2() had a redundant (y != 0) check that is not optimized out.
x86-64 clang with "-Os" option (tested in Compiler Explorer, a.k.a. godbolt.org)
```x86asm
func1:
movq %rsi, %rax
mulq %rdi
seto %al
retq
func2:
testq %rsi, %rsi
je .LBB1_1
movq %rsi, %rax
mulq %rdi
seto %al
retq
.LBB1_1:
xorl %eax, %eax
retq
```
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