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Meta: Globally disable floating point contraction

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FP contraction is a standard-conforming behavior which allows the
compiler to calculate intermediate results of expressions containing
floating point numbers with a greater precision than the expression type
allows. And in theory, it enables additional optimizations, such as
replacing `a * b + c` with fma(a, b, c).

Unfortunately, it is extremely hard to predict when the contraction will
happen. For example, Clang 17 on x86_64 with the default options will
use FMA only for constant-folded non-constexpr expressions. So, in
practice, FP contraction leads to hard-to-find bugs and inconsistencies
between executables compiled with different toolchains or for different
OSes. And we had two instances of this happening last week.

Since we did not ever used -mfma on x86_64, this patch can only possibly
regress performance on Apple ARM devices, where FMA is enabled by
default. However, this regression will likely be negligible since the
difference would be one additional add instruction, which would be then
likely executed in parallel with something else.
This commit is contained in:
Dan Klishch 2023-11-16 14:48:32 -05:00 committed by Tim Flynn
parent 67bc7cecbb
commit 2dba79bc6b
Notes: sideshowbarker 2024-07-16 23:51:07 +09:00 
Author: https://github.com/DanShaders
Commit: https://github.com/SerenityOS/serenity/commit/2dba79bc6b
Pull-request: https://github.com/SerenityOS/serenity/pull/21963
Reviewed-by: https://github.com/trflynn89 ✅
2 changed files with 4 additions and 0 deletions
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2
Meta/CMake/common_compile_options.cmake
View File

@ -12,6 +12,8 @@ add_compile_options(-Wno-unused-command-line-argument)
add_compile_options(-fdiagnostics-color=always)
add_compile_options(-fno-exceptions)
add_compile_options(-ffp-contract=off)
if (NOT CMAKE_HOST_SYSTEM_NAME MATCHES SerenityOS)
# FIXME: Something makes this go crazy and flag unused variables that aren't flagged as such when building with the toolchain.
# Disable -Werror for now.

2
Meta/gn/build/BUILD.gn
View File

@ -84,6 +84,8 @@ config("compiler_defaults") {
}
cflags += [ "-fdiagnostics-color" ]
cflags += [ "-ffp-contract=off" ]
if (use_lld) {
ldflags += [ "-Wl,--color-diagnostics" ]
}