Adding -ffp-contract=off flag.

This is needed for floating point stability. The global placement depends on the output of the FFT library. Without `-ffp-contract=off` the current FFT library produces different results dependent on if the compiler is targeting an x86 architecture which has the "FMA instruction". The FMA instruction omits the rounding done in the multiplication instruction, making the calculations produce different results for some input values See https://kristerw.github.io/2021/11/09/fp-contract/ See also OpenROAD change @ The-OpenROAD-Project/OpenROAD#4518 Signed-off-by: Tim 'mithro' Ansell <[email protected]>
mithro · Jan 10, 2024 · 0f59694 · 0f59694
1 parent 14c1dfd
commit 0f59694
Showing 1 changed file with 1 addition and 0 deletions.
diff --git a/dependency_support/org_theopenroadproject/build_helper.bzl b/dependency_support/org_theopenroadproject/build_helper.bzl
@@ -92,6 +92,7 @@ OPENROAD_BINARY_SRCS = OPENROAD_BINARY_SRCS_WITHOUT_MAIN + [
 
 OPENROAD_COPTS = [
     "-fexceptions",
+    "-ffp-contract=off", # Needed for floating point stability.
     "-Wno-error",
     "-Wall",
     "-Wextra",