<table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Issue</th>
<td>
<a href=https://github.com/llvm/llvm-project/issues/118265>118265</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>
[clang] `-O2` requires `-ftrapping-math` or `-ffast-math` on amd64 to avoid false positives with `feenableexcept(FE_DIVBYZERO)`
</td>
</tr>
<tr>
<th>Labels</th>
<td>
clang
</td>
</tr>
<tr>
<th>Assignees</th>
<td>
</td>
</tr>
<tr>
<th>Reporter</th>
<td>
illwieckz
</td>
</tr>
</table>
<pre>
Hi, using this sample code:
```c++
#include <cfenv>
#include <cstdio>
struct bc_t
{
float minDist, maxDist;
float hudSize;
float a1, a2, a3 = 0.0f;
float minSize;
float b1, b2, b3 = 0.0f;
float maxSize;
float c1, c2, c3 = 0.0f;
};
int main(int, char**)
{
feenableexcept(FE_DIVBYZERO);
bc_t bc;
sscanf("1 1 1", "%f %f %f", &bc.hudSize, &bc.minSize, &bc.maxSize);
bc.minDist = bc.hudSize / bc.maxSize;
bc.maxDist = bc.hudSize / bc.minSize;
printf("%f\n", bc.minDist);
return 0;
}
```
By compiling it with `-O2`, the compiled code raises some `division by zero` exception when executed:
```
Process 3247973 stopped
* thread #1, name = 'a.out', stop reason = signal SIGFPE: floating point divide by zero
frame #0: 0x00005555555551c5 a.out`main + 101
a.out`main:
-> 0x5555555551c5 <+101>: divps %xmm2, %xmm0
0x5555555551c8 <+104>: movlps %xmm0, (%rsp)
0x5555555551cc <+108>: cvtss2sd %xmm0, %xmm0
0x5555555551d0 <+112>: leaq 0xe3c(%rip), %rdi
Process 3249378 stopped
* thread #1, name = 'a.out', stop reason = signal SIGFPE: floating point divide by zero
frame #0: 0x00005555555551c5 a.out`main((null)=<unavailable>, (null)=<unavailable>) at a.cpp:20:26
17 feenableexcept(FE_DIVBYZERO);
18 bc_t bc;
19 sscanf("1 1 1", "%f %f %f", &bc.hudSize, &bc.maxSize, &bc.minSize);
-> 20 bc.minDist = bc.hudSize / bc.maxSize;
21 bc.maxDist = bc.hudSize / bc.minSize;
22 printf("%f\n", bc.minDist);
23 return 0;
```
My guess is that the SSE optimized code raises division by zero errors from unused fields of the xmm registers, despite we discard those results.
The requirement of `-O2` requiring either `-ftrapping-math` or `-ffast-math` is curious since `-ffast-math` sets `-fno-trapping-math`.
Also, I thought `-ftrapping-math` was the default, and that `-fno-trapping-math` is part of `-ffast-math`, but just using `-O2` behaves like if `-fno-trapping-math` is used, so it behaves like if part of `-ffast-math` was enabled anyway… But maybe `-ffast-math` just modifies other behaviors that make `-ftrapping-math` or `-fno-trapping-math` irrelevant.
Using the Godbolt's compiler explorer (here Clang 19.1.0), it only works with `-O2 -ffast-math` (or depreacted `-Ofast`):
|compiler flags|status|
|-|-|
||✅️|
|`-O0`|✅️|
|`-Os`|❌️|
|`-Os -fno-trapping-math`|❌️|
|`-Os -ftrapping-math`|✅️|
|`-O1`|✅️|
|`-O2`|❌️|
|`-O2 -fno-trapping-math`|❌️|
|`-O2 -ftrapping-math`|✅️|
|`-ffast-math`|✅️|
|`-O2 -ffast-math`|✅️|
|`-O2 -ffast-math -ftrapping-math`|✅️|
|`-O2 -ffast-math -fno-trapping-math`|✅️|
|`-Ofast`|✅️|
See: https://godbolt.org/z/395c8nMef
And (with `-ftrapping-math` added): https://godbolt.org/z/czbGTjs6E
And (with `-fno-trapping-math` added): https://godbolt.org/z/zYr5Pba3W
With 32-bit i686 I reproduce the bug when using SSE but get no bug when not using SSE.
|compiler flags|`-m32 -msse`|`-m32 -mno-sse`|
|-|-|-|
||✅️|✅️|
|`-Os`|❌️|✅️|
|`-O1`|✅️|✅️|
|`-O2`|❌️|✅️|
|`-ffast-math`|✅️|✅️|
|`-O2 -ffast-math`|✅️|✅️|
See 32-bit i686 with SSE I get the same failure: https://godbolt.org/z/199eqhzGh
And 32-bit i686 build without SSE I get no failure: https://godbolt.org/z/sfd9TsTj4
On my end on Ubuntu 24.04 with amd64, I get same results with clang 19.1.5.
On Ubuntu 24.04 with amd64 and different versions of the clang compiler, I only get it working with clang 13 and 14, every later version breaks it:
||`-O0`|`-O1`|`-O2`|`-O2 -ftrapping-math`|`-ffast-math`|`-O2 -ffast-math`|
|-|-|-|-|-|-|-|
|clang 13.0.1|✅️|✅️|✅️|✅️|✅️|✅️|
|clang 14.0.6|✅️|✅️|✅️|✅️|✅️|✅️|
|clang 15.0.7|✅️|✅️|❌️|✅️|✅️|✅️|
|clang 16.0.6|✅️|✅️|❌️|✅️|✅️|✅️|
|clang 17.0.6|✅️|✅️|❌️|✅️|✅️|✅️|
|clang 18.1.3|✅️|✅️|❌️|✅️|✅️|✅️|
|clang 19.1.5|✅️|✅️|❌️|✅️|✅️|✅️|
On GCC 14.02 I get none of those issues (no one false positive `division by zero` error is raised whatever the compiler flags being used: https://godbolt.org/z/rxMWM68Mc
___
_Note:_ Disabling SSE on amd64 just produces garbage computation (`1.0/1.0` gives `0.0`), but I don't know if that makes sense to disable SSE on amd64. GCC produces the same garbage (`1.0/1.0` giving `0.0`). Though I'm surprised to not get a warning if that's not legit to do, also I'm surprised the generated code runs if that's not legit to do. See: https://godbolt.org/z/77PYGTc5b (Clang) and: https://godbolt.org/z/cvW4Er3Kd (GCC).
</pre>
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