[compiler-rt] [compiler-rt][ARM] Optimized f32 add/subtract for Armv6-M. (PR #154093)

[Peter Smith via llvm-commits]

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@@ -0,0 +1,29 @@
+//===-- 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.
+//
+//===----------------------------------------------------------------------===//
+
+unsigned __fnan2(unsigned a, unsigned b) {
+  unsigned aadj = (a << 1) + 0x00800000;
+  unsigned badj = (b << 1) + 0x00800000;
+  if (aadj > 0xff800000)
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smithp35 wrote:

Is it worth accommodating some of the additional comments in https://github.com/ARM-software/optimized-routines/blob/master/fp/common/dnan2.c to make it easier to recognize the bit-patterns?

https://github.com/llvm/llvm-project/pull/154093