[clang][FatLTO][UnifiedLTO] Pass -enable-matrix to the LTO driver

[ormris]

Unified LTO and Fat LTO do not use the regular LTO prelink pipeline when -flto/-flto=full is specified on the command line, thus they require LowerMatrixIntrinsicsPass to be run during the link stage. To enable this, we pass -enable-matrix to the LTO driver, replicating ThinLTO behavior.

This fixes #77621.

[llvmbot]

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-driver

Author: Matthew Voss (ormris)

Changes

Unified LTO and Fat LTO do not use the regular LTO prelink pipeline when -flto/-flto=full is specified on the command line, thus they require LowerMatrixIntrinsicsPass to be run during the link stage. To enable this, we pass -enable-matrix to the LTO driver, replicating ThinLTO behavior.

This fixes #77621.

---

Full diff: https://github.com/llvm/llvm-project/pull/77829.diff

2 Files Affected:

• (modified) clang/lib/Driver/ToolChains/CommonArgs.cpp (+5-2)
• (modified) clang/test/Driver/matrix.c (+12)

diff --git a/clang/lib/Driver/ToolChains/CommonArgs.cpp b/clang/lib/Driver/ToolChains/CommonArgs.cpp
index 2340191ca97d98..385f66f3782bc1 100644
--- a/clang/lib/Driver/ToolChains/CommonArgs.cpp
+++ b/clang/lib/Driver/ToolChains/CommonArgs.cpp
@@ -736,6 +736,8 @@ void tools::addLTOOptions(const ToolChain &ToolChain, const ArgList &Args,
   const bool IsAMDGCN = ToolChain.getTriple().isAMDGCN();
   const char *Linker = Args.MakeArgString(ToolChain.GetLinkerPath());
   const Driver &D = ToolChain.getDriver();
+  const bool IsFatLTO = Args.hasArg(options::OPT_ffat_lto_objects);
+  const bool IsUnifiedLTO = Args.hasArg(options::OPT_funified_lto);
   if (llvm::sys::path::filename(Linker) != "ld.lld" &&
       llvm::sys::path::stem(Linker) != "ld.lld" &&
       !ToolChain.getTriple().isOSOpenBSD()) {
@@ -765,7 +767,7 @@ void tools::addLTOOptions(const ToolChain &ToolChain, const ArgList &Args,
   } else {
     // Tell LLD to find and use .llvm.lto section in regular relocatable object
     // files
-    if (Args.hasArg(options::OPT_ffat_lto_objects))
+    if (IsFatLTO)
       CmdArgs.push_back("--fat-lto-objects");
   }
 
@@ -825,7 +827,8 @@ void tools::addLTOOptions(const ToolChain &ToolChain, const ArgList &Args,
   // Matrix intrinsic lowering happens at link time with ThinLTO. Enable
   // LowerMatrixIntrinsicsPass, which is transitively called by
   // buildThinLTODefaultPipeline under EnableMatrix.
-  if (IsThinLTO && Args.hasArg(options::OPT_fenable_matrix))
+  if ((IsThinLTO || IsFatLTO || IsUnifiedLTO) &&
+        Args.hasArg(options::OPT_fenable_matrix))
     CmdArgs.push_back(
         Args.MakeArgString(Twine(PluginOptPrefix) + "-enable-matrix"));
 
diff --git a/clang/test/Driver/matrix.c b/clang/test/Driver/matrix.c
index 15b44ce5a4ec15..4d2624ad39c16e 100644
--- a/clang/test/Driver/matrix.c
+++ b/clang/test/Driver/matrix.c
@@ -6,3 +6,15 @@
 // RUN: %clang -flto=thin -fenable-matrix %t.o -### --target=x86_64-unknown-linux 2>&1 \
 // RUN:     | FileCheck %s -check-prefix=CHECK-THINLTO-MATRIX
 // CHECK-THINLTO-MATRIX: "-plugin-opt=-enable-matrix"
+
+// RUN: %clang -flto -ffat-lto-objects -fenable-matrix %t.o -### --target=x86_64-unknown-linux 2>&1 \
+// RUN:     | FileCheck %s -check-prefix=CHECK-THINLTO-MATRIX
+
+// RUN: %clang -flto=thin -ffat-lto-objects -fenable-matrix %t.o -### --target=x86_64-unknown-linux 2>&1 \
+// RUN:     | FileCheck %s -check-prefix=CHECK-THINLTO-MATRIX
+
+// RUN: %clang -flto -funified-lto -fenable-matrix %t.o -### --target=x86_64-unknown-linux 2>&1 \
+// RUN:     | FileCheck %s -check-prefix=CHECK-THINLTO-MATRIX
+
+// RUN: %clang -flto=thin -funified-lto -fenable-matrix %t.o -### --target=x86_64-unknown-linux 2>&1 \
+// RUN:     | FileCheck %s -check-prefix=CHECK-THINLTO-MATRIX

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff 2bb511e277e501d3faa0f2da0d1c98ea0b515507 b8d3ec5cde2b311747aec83d316af370202b37c1 -- clang/lib/Driver/ToolChains/CommonArgs.cpp clang/test/Driver/matrix.c

View the diff from clang-format here.

diff --git a/clang/lib/Driver/ToolChains/CommonArgs.cpp b/clang/lib/Driver/ToolChains/CommonArgs.cpp
index 385f66f378..6305085087 100644
--- a/clang/lib/Driver/ToolChains/CommonArgs.cpp
+++ b/clang/lib/Driver/ToolChains/CommonArgs.cpp
@@ -828,7 +828,7 @@ void tools::addLTOOptions(const ToolChain &ToolChain, const ArgList &Args,
   // LowerMatrixIntrinsicsPass, which is transitively called by
   // buildThinLTODefaultPipeline under EnableMatrix.
   if ((IsThinLTO || IsFatLTO || IsUnifiedLTO) &&
-        Args.hasArg(options::OPT_fenable_matrix))
+      Args.hasArg(options::OPT_fenable_matrix))
     CmdArgs.push_back(
         Args.MakeArgString(Twine(PluginOptPrefix) + "-enable-matrix"));
 

[MaskRay]

The description should mention https://reviews.llvm.org/D153583 ("[ThinLTO][Matrix] Forward -enable-matrix flag to the LTO plugin"). I think this is correct when used in the ThinLTO mode, but it's not clear whether regular LTO mode is correct (perhaps correct but redundant).

[ormris]

Thanks! I'll make sure to include that review in the commit message. Unified LTO is using the ThinLTO pre-link pipeline, regardless of what's passed to the driver. Since the ThinLTO pre-link pipeline doesn't provide this pass, this option should be required when Unified LTO is used.