Add -ffp-contract=off compile flag.

GCC per default enables one optimization for x86_64 that can change the
result of floating-point operations: -ffp-contract=fast.

This allows the compiler to do floating-point expression contraction
such as combining multiplication and addition instructions with an FMA
instruction. However, the FMA instruction omits the rounding done in the
multiplication instruction, making the calculations produce different
results for some input values.

Disabling floating-point contraction with -ffp-contract=off makes GCC
generate code that produces a consistent result for x86_64.

See https://kristerw.github.io/2021/11/09/fp-contract/

The `fft_test` checks that this is working correctly.

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>

src/CMakeLists.txt

@@ -192,6 +192,9 @@ add_compile_options(
   -Wno-sign-compare
   -Wp,-D_GLIBCXX_ASSERTIONS
   $<$<CXX_COMPILER_ID:Clang>:-Wno-gnu-zero-variadic-macro-arguments>
+  # Needed for floating point stability in FFT (fft_test will check this).
+  # See also https://kristerw.github.io/2021/11/09/fp-contract/
+  -ffp-contract=off
   # Apple clang 14.0.0 deprecates sprintf, which generates 900 warnings.
   $<$<CXX_COMPILER_ID:AppleClang>:-Wno-deprecated-declarations>
   $<$<BOOL:${ASAN}>:-fsanitize=address>