[libc][math] Fix setting exceptional value for tanf to work with gcc.

[lntue]

Summary

See https://github.com/llvm/llvm-project/issues/59866

[sivachandra]

libc/src/math/generic/tanf.cpp

100–105

Can you add some commentary to explain why a volatile is required?

[lntue]

Add comments.

libc/src/math/generic/tanf.cpp

#endif // LIBC_TARGET_HAS_FMA
     return xd * result;
   }
 
   // Check for exceptional values
   if (unlikely(x_abs == 0x3f8a1f62U)) {
     // |x| = 0x1.143ec4p0
     float sign = x_sign ? -1.0f : 1.0f;
 
-    return fputil::multiply_add(sign, 0x1.ddf9f4p0f, sign * 0x1.1p-24f);
+    // volatile is used to prevent compiler (gcc) from optimizing the
+    // computation, making the results incorrect in different rounding modes.
+    volatile float tmp = 0x1.ddf9f4p0f;
+    tmp = fputil::multiply_add(sign, tmp, sign * 0x1.1p-24f);
+
+    return tmp;
   }
 
   // |x| > 0x1.ada6a8p+27f
   if (unlikely(x_abs > 0x4d56'd354U)) {
     // Inf or NaN
     if (unlikely(x_abs >= 0x7f80'0000U)) {
       if (x_abs == 0x7f80'0000U) {
         errno = EDOM;