bazel: move -fopenmp to global .bazelrc, fixes The-OpenROAD-Project#7451

Move -fopenmp from per-target copts to a global --cxxopt in .bazelrc
so all OpenROAD code is compiled with OpenMP support without needing
it in each BUILD file.

External BCR deps (e.g. or-tools, eigen) get -fno-openmp via
--per_file_copt to avoid omp.h not found errors, since @OpenMP
headers aren't in their sandbox and toolchains_llvm doesn't yet
expose them (see bazel-contrib/toolchains_llvm#339).

Note: src/sta (OpenSTA submodule) still has a per-target -fopenmp
that should be removed in a follow-up PR to that repo.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: oharboe

.bazelrc

@@ -32,6 +32,13 @@ build:opt --linkopt=-flto
 # See also https://kristerw.github.io/2021/11/09/fp-contract/
 build --cxxopt "-ffp-contract=off" --host_cxxopt "-ffp-contract=off"
 
+# Use OpenMP for all OpenROAD code. External BCR deps (e.g. or-tools, eigen)
+# can't use it yet because the @openmp headers aren't in their sandbox and
+# toolchains_llvm doesn't expose omp.h (see bazel-contrib/toolchains_llvm#339).
+build --cxxopt "-fopenmp"               --host_cxxopt "-fopenmp"
+build --per_file_copt=external/.*@-fno-openmp
+build --host_per_file_copt=external/.*@-fno-openmp
+
 # allow exceptions.
 build  --cxxopt=-fexceptions    --host_cxxopt=-fexceptions
 

BUILD.bazel

@@ -145,7 +145,6 @@ OPENROAD_COPTS = [
     "-Wno-sign-compare",
     "-Wno-unused-parameter",
     "-Wmismatched-tags",
-    "-fopenmp",
 ]
 
 OPENROAD_DEFINES = [

src/ant/BUILD

@@ -25,9 +25,6 @@ cc_library(
     hdrs = [
         "include/ant/AntennaChecker.hh",
     ],
-    copts = [
-        "-fopenmp",
-    ],
     includes = [
         "include",
     ],

src/drt/BUILD

@@ -199,7 +199,6 @@ cc_library(
     ],
     copts = [
         "-Isrc/drt/src",
-        "-fopenmp",
     ],
     includes = [
         "include",

src/est/BUILD

@@ -73,7 +73,6 @@ cc_library(
     copts = [
         "-Isrc/est/src",
         "-Wno-missing-braces",  # from TCL swigging
-        "-fopenmp",
     ],
     includes = [
         "include",

src/gpl/BUILD

@@ -52,9 +52,6 @@ cc_library(
         "src/graphicsNone.h",
         "src/mbff.h",
     ],
-    copts = [
-        "-fopenmp",
-    ],
     includes = [
         "include",
     ],

src/grt/BUILD

@@ -133,9 +133,6 @@ cc_library(
         "include/grt/RoutePt.h",
         "include/grt/Rudy.h",
     ],
-    copts = [
-        "-fopenmp",
-    ],
     includes = [
         "include",
     ],
@@ -186,7 +183,6 @@ cc_library(
     copts = [
         "-Isrc/grt/src",
         "-Wno-missing-braces",  # from TCL swigging
-        "-fopenmp",
     ],
     includes = [
         "include",