<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/82857>82857</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            "float_control(precise, on)" no longer has an effect if a function is inlined
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang:frontend,
            floating-point
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          ormris
      </td>
    </tr>
</table>

<pre>
    The `float_control(precise, on)` no longer has an effect if a function is inlined. In the example below, adding the "noinline" attribute to a function changes the instruction used in the division (vdivss vs vrcpps).

```bash
$ cat inline.cpp
#pragma float_control(push)
#pragma float_control(precise, on)
float e(float a, float b)
{
    return a / b;
}
#pragma float_control(pop)

struct c { float a, b, c, d; };

__attribute__((noinline)) c
b(float X, float Y)
{
 return c {
        2.0f * e(0.0f, X) - 1.0f,
        1.0f - 2.0f * e(3.0f, Y),
        2.0f * e(4.0f, X) - 1.0f,
        1.0f - 2.0f * e(5.0f, Y)
    };
}

int main()
{
    c f = b(1.0f, 2.0f);
    return (int) f.a + f.b + f.c + f.d;
}
$ diff -u -d inline.cpp noinline.cpp
--- inline.cpp  2024-02-23 17:26:53.041466900 -0800
+++ noinline.cpp        2024-02-23 17:27:16.320344400 -0800
@@ -1,5 +1,6 @@
 #pragma float_control(push)
 #pragma float_control(precise, on)
+__attribute__((noinline))
 float e(float a, float b)
 {
     return a / b;
$ clang -S -O3 -ffast-math inline.cpp
$ clang -S -O3 -ffast-math noinline.cpp
$ cat inline.s
...
_Z1bff:                                 # @_Z1bff
...
        vmovss  .LCPI1_2(%rip), %xmm3           # xmm3 = [-1.0E+0,0.0E+0,0.0E+0,0.0E+0]
        vdivss  %xmm1, %xmm2, %xmm2
        vrcpps  %xmm0, %xmm1 # <----------
        vaddss  .LCPI1_1(%rip), %xmm2, %xmm2
        vmulps  %xmm4, %xmm1, %xmm5
...
$ cat noinline.s
...
_Z1eff:                                 # @_Z1eff
        .cfi_startproc
# %bb.0:                                # %entry
        vdivss  %xmm1, %xmm0, %xmm0 # <----------
        retq
...
$
```

</pre>
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