[OpenMP][AIX] Add pthreads and perfstat libraries for linking static libomp.a #167706

kkwli · 2025-11-12T15:51:03Z

The libpthreads and perfstat libraries are required on AIX if the libomp.a is a static library.

…libomp

llvmbot · 2025-11-12T15:51:35Z

@llvm/pr-subscribers-backend-powerpc

@llvm/pr-subscribers-clang

Author: Kelvin Li (kkwli)

Changes

The libpthreads and perfstat libraries are required on AIX if the libomp.a is a static library.

Full diff: https://github.com/llvm/llvm-project/pull/167706.diff

1 Files Affected:

(modified) clang/lib/Driver/ToolChains/AIX.cpp (+8-3)

diff --git a/clang/lib/Driver/ToolChains/AIX.cpp b/clang/lib/Driver/ToolChains/AIX.cpp
index a8acf9cfc44c9..ad32b4f5b16b6 100644
--- a/clang/lib/Driver/ToolChains/AIX.cpp
+++ b/clang/lib/Driver/ToolChains/AIX.cpp
@@ -309,8 +309,10 @@ void aix::Linker::ConstructJob(Compilation &C, const JobAction &JA,
       AddRunTimeLibs(ToolChain, D, CmdArgs, Args);
 
       // Add OpenMP runtime if -fopenmp is specified.
-      if (Args.hasFlag(options::OPT_fopenmp, options::OPT_fopenmp_EQ,
-                       options::OPT_fno_openmp, false)) {
+      const bool hasFOpenMP =
+          Args.hasFlag(options::OPT_fopenmp, options::OPT_fopenmp_EQ,
+                       options::OPT_fno_openmp, false);
+      if (hasFOpenMP) {
         switch (ToolChain.getDriver().getOpenMPRuntime(Args)) {
         case Driver::OMPRT_OMP:
           CmdArgs.push_back("-lomp");
@@ -325,10 +327,13 @@ void aix::Linker::ConstructJob(Compilation &C, const JobAction &JA,
           // Already diagnosed.
           break;
         }
+
+        CmdArgs.push_back("-lperfstat");
       }
 
       // Support POSIX threads if "-pthreads" or "-pthread" is present.
-      if (Args.hasArg(options::OPT_pthreads, options::OPT_pthread))
+      if (hasFOpenMP ||
+          Args.hasArg(options::OPT_pthreads, options::OPT_pthread))
         CmdArgs.push_back("-lpthreads");
 
       if (D.CCCIsCXX())

llvmbot · 2025-11-12T15:51:35Z

@llvm/pr-subscribers-clang-driver

Author: Kelvin Li (kkwli)

Changes

The libpthreads and perfstat libraries are required on AIX if the libomp.a is a static library.

Full diff: https://github.com/llvm/llvm-project/pull/167706.diff

1 Files Affected:

(modified) clang/lib/Driver/ToolChains/AIX.cpp (+8-3)

diff --git a/clang/lib/Driver/ToolChains/AIX.cpp b/clang/lib/Driver/ToolChains/AIX.cpp
index a8acf9cfc44c9..ad32b4f5b16b6 100644
--- a/clang/lib/Driver/ToolChains/AIX.cpp
+++ b/clang/lib/Driver/ToolChains/AIX.cpp
@@ -309,8 +309,10 @@ void aix::Linker::ConstructJob(Compilation &C, const JobAction &JA,
       AddRunTimeLibs(ToolChain, D, CmdArgs, Args);
 
       // Add OpenMP runtime if -fopenmp is specified.
-      if (Args.hasFlag(options::OPT_fopenmp, options::OPT_fopenmp_EQ,
-                       options::OPT_fno_openmp, false)) {
+      const bool hasFOpenMP =
+          Args.hasFlag(options::OPT_fopenmp, options::OPT_fopenmp_EQ,
+                       options::OPT_fno_openmp, false);
+      if (hasFOpenMP) {
         switch (ToolChain.getDriver().getOpenMPRuntime(Args)) {
         case Driver::OMPRT_OMP:
           CmdArgs.push_back("-lomp");
@@ -325,10 +327,13 @@ void aix::Linker::ConstructJob(Compilation &C, const JobAction &JA,
           // Already diagnosed.
           break;
         }
+
+        CmdArgs.push_back("-lperfstat");
       }
 
       // Support POSIX threads if "-pthreads" or "-pthread" is present.
-      if (Args.hasArg(options::OPT_pthreads, options::OPT_pthread))
+      if (hasFOpenMP ||
+          Args.hasArg(options::OPT_pthreads, options::OPT_pthread))
         CmdArgs.push_back("-lpthreads");
 
       if (D.CCCIsCXX())

DanielCChen · 2025-11-12T17:01:31Z

clang/lib/Driver/ToolChains/AIX.cpp

          break;
        }
+
+        CmdArgs.push_back("-lperfstat");


How to tell if this is only added for static libomp.a?

Yeah, this is going to add it for the dynamic links as well, I don't know if that's a behaviour we want.

@DanielCChen @daltenty
The situation is that libomp.a is a static library (i.e. ar t shows only .o) and we link it by default (i.e. -by option). It fails with missing pthread_* and ptx_* symbols. Adding the two libraries resolves the issues. Before and after the patch, ldd shows:

/usr/lib/libpthreads.a(shr_xpg5_64.o) /usr/lib/libperfstat.a(shr_64.o)

If I understand correctly, there is not doubt that we need this IF the libomp.a is built as a static library. I think the question is if libomp.a is built as a shared library, we probably don't want to link to -lperfstat, and this PR doesn't seem differentiate that.

If the shared libomp.a is built (i.e. -DLIBOMP_ENABLE_SHARED=ON), it contains only libomp.so.1 as returned by at t. The shared library also has dependency on libperfstat.a.

$ dump -H ./runtimes/runtimes-bins/openmp/runtime/src/libomp.a ./runtimes/runtimes-bins/openmp/runtime/src/libomp.a[libomp.so.1]: ***Loader Section*** ... ***Import File Strings*** INDEX PATH BASE MEMBER 0 /home/kli/wrk/f/build-flang/lib/clang/22/lib/powerpc64-ibm-aix:/usr/lib:/lib: 1 libc.a shr_64.o 2 libpthreads.a shr_xpg5_64.o 3 libperfstat.a shr_64.o

For linking this shared library (libomp.a), adding -lperfstat will not change the behavior as far as I know.

For dynamic linking, there is no extra dependence based on the output of ldd.

Without -lperfstat,

$ /usr/bin/ld -o a.out -b64 -bpT:0x100000000 -bpD:0x110000000 /usr/lib/crt0_64.o /usr/lib/crti_64.o -bcdtors:all:0:s parallel.o -L/home/kli/llvm-install/bin/../lib/powerpc64-ibm-aix /home/kli/llvm-install/lib/clang/22/lib/powerpc64-ibm-aix/libclang_rt.builtins.a -lunwind -lomp -lc $ ldd a.out a.out needs: /usr/lib/libc.a(shr_64.o) /home/kli/llvm-install/bin/../lib/powerpc64-ibm-aix/libomp.a(libomp.so.1) /unix /usr/lib/libcrypt.a(shr_64.o) /usr/lib/libpthreads.a(shr_xpg5_64.o) /usr/lib/libperfstat.a(shr_64.o) /usr/lib/libcfg.a(shr_64.o) /usr/lib/libodm.a(shr_64.o) /usr/lib/liblvm.a(shr_64.o) /usr/lib/libcorcfg.a(shr_64.o) /usr/lib/libsrc.a(shr_64.o)

With -lperfstat,

$ /usr/bin/ld -o a.out -b64 -bpT:0x100000000 -bpD:0x110000000 /usr/lib/crt0_64.o /usr/lib/crti_64.o -bcdtors:all:0:s parallel.o -L/home/kli/llvm-install/bin/../lib/powerpc64-ibm-aix /home/kli/llvm-install/lib/clang/22/lib/powerpc64-ibm-aix/libclang_rt.builtins.a -lunwind -lomp -lc -lperfstat $ ldd a.out a.out needs: /usr/lib/libc.a(shr_64.o) /home/kli/llvm-install/bin/../lib/powerpc64-ibm-aix/libomp.a(libomp.so.1) /unix /usr/lib/libcrypt.a(shr_64.o) /usr/lib/libpthreads.a(shr_xpg5_64.o) /usr/lib/libperfstat.a(shr_64.o) /usr/lib/libcfg.a(shr_64.o) /usr/lib/libodm.a(shr_64.o) /usr/lib/liblvm.a(shr_64.o) /usr/lib/libcorcfg.a(shr_64.o) /usr/lib/libsrc.a(shr_64.o)

clang/lib/Driver/ToolChains/AIX.cpp

daltenty · 2025-11-12T17:54:09Z

clang/lib/Driver/ToolChains/AIX.cpp

          break;
        }
+
+        CmdArgs.push_back("-lperfstat");


Yeah, this is going to add it for the dynamic links as well, I don't know if that's a behaviour we want.

clang/lib/Driver/ToolChains/AIX.cpp

kkwli · 2025-11-13T19:34:53Z

If the static libomp.a library is built, users need to explicitly specify -lpthreads and -lperfstat. I withdraw this PR.

[OpenMP][AIX] Add pthreads and perfstat libraries for linking static …

e23c895

…libomp

kkwli requested review from DanielCChen and xingxue-ibm November 12, 2025 15:51

kkwli self-assigned this Nov 12, 2025

llvmbot added clang Clang issues not falling into any other category backend:PowerPC clang:driver 'clang' and 'clang++' user-facing binaries. Not 'clang-cl' labels Nov 12, 2025

DanielCChen reviewed Nov 12, 2025

View reviewed changes

daltenty requested changes Nov 12, 2025

View reviewed changes

avoid duplication of -lpthreads in flang

6f8ea42

DanielCChen reviewed Nov 13, 2025

View reviewed changes

clang/lib/Driver/ToolChains/AIX.cpp Show resolved Hide resolved

kkwli closed this Nov 13, 2025

kkwli deleted the add-pthread branch November 13, 2025 19:35

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[OpenMP][AIX] Add pthreads and perfstat libraries for linking static libomp.a #167706

[OpenMP][AIX] Add pthreads and perfstat libraries for linking static libomp.a #167706

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[OpenMP][AIX] Add pthreads and perfstat libraries for linking static libomp.a #167706

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