[compiler-rt] [compiler-rt][ARM] Optimized mulsf3 and divsf3 (PR #161546)

[Saleem Abdulrasool via llvm-commits]

================
@@ -0,0 +1,38 @@
+//===-- fnan2.c - Handle single-precision NaN inputs to binary operation --===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This helper function is available for use by single-precision float
+// arithmetic implementations to handle propagating NaNs from the input
+// operands to the output, in a way that matches Arm hardware FP.
+//
+// On input, a and b are floating-point numbers in IEEE 754 encoding, and at
+// least one of them must be a NaN. The return value is the correct output NaN.
+//
+//===----------------------------------------------------------------------===//
+
+#include <stdint.h>
+
+uint32_t __compiler_rt_fnan2(uint32_t a, uint32_t b) {
+  // Make shifted-left copies of a and b to discard the sign bit. Then add 1 at
+  // the bit position where the quiet vs signalling bit ended up. This squashes
+  // all the signalling NaNs to the top of the range of 32-bit values, from
+  // 0xff800001 to 0xffffffff inclusive; meanwhile, all the quiet NaN values
+  // wrap round to the bottom, from 0 to 0x007fffff inclusive. So we can detect
+  // a signalling NaN by asking if it's greater than 0xff800000, and a quiet
+  // one by asking if it's less than 0x00800000.
+  uint32_t aadj = (a << 1) + 0x00800000;
+  uint32_t badj = (b << 1) + 0x00800000;
+  if (aadj > 0xff800000)   // a is a signalling NaN?
+    return a | 0x00400000; //   if so, return it with the quiet bit set
+  if (badj > 0xff800000)   // b is a signalling NaN?
+    return b | 0x00400000; //   if so, return it with the quiet bit set
+  if (aadj < 0x00800000)   // a is a quiet NaN?
+    return a;              // if so, return it
+  else                     // expect (badj < 0x00800000)
+    return b;              // in that case b must be a quiet NaN
----------------
compnerd wrote:

I think that this should be either of the following:

```
return (aadj < 0x00800000) ? a : b;
```

or

```
if (aadj < 0x00800000)
  return a;
return b;
```

https://github.com/llvm/llvm-project/pull/161546