[LLVM] Add 'ExpandVariadicsPass' to LTO default pipeline

Summary:
This pass expands variadic functions into non-variadic function calls
according to the target ABI. Currently, this is used as the lowering for
the NVPTX, AMDGPU, and WASM targets.

This pass is currently only run late in the target's backend. However,
during LTO we want to run it before the inliner pass so that the
expanded functions can be inlined using standard heuristics. This pass
is a no-op for unsupported targets, so this won't apply to any code that
isn't already using it.

Author: jhuber6

llvm/lib/Passes/PassBuilderPipelines.cpp

@@ -48,6 +48,7 @@
 #include "llvm/Transforms/IPO/DeadArgumentElimination.h"
 #include "llvm/Transforms/IPO/ElimAvailExtern.h"
 #include "llvm/Transforms/IPO/EmbedBitcodePass.h"
+#include "llvm/Transforms/IPO/ExpandVariadics.h"
 #include "llvm/Transforms/IPO/ForceFunctionAttrs.h"
 #include "llvm/Transforms/IPO/FunctionAttrs.h"
 #include "llvm/Transforms/IPO/GlobalDCE.h"
@@ -1874,6 +1875,9 @@ PassBuilder::buildLTODefaultPipeline(OptimizationLevel Level,
   MPM.addPass(createModuleToFunctionPassAdaptor(std::move(PeepholeFPM),
                                                 PTO.EagerlyInvalidateAnalyses));
 
+  // Lower variadic functions for supported targets prior to inlining.
+  MPM.addPass(ExpandVariadicsPass(ExpandVariadicsMode::Optimize));
+
   // Note: historically, the PruneEH pass was run first to deduce nounwind and
   // generally clean up exception handling overhead. It isn't clear this is
   // valuable as the inliner doesn't currently care whether it is inlining an

llvm/test/Other/new-pm-lto-defaults.ll

@@ -29,6 +29,7 @@
 
 ; CHECK-EP: Running pass: NoOpModulePass
 ; CHECK-O: Running pass: CrossDSOCFIPass
+; CHECK-O-NEXT: Running pass: ExpandVariadicsPass
 ; CHECK-O-NEXT: Running pass: OpenMPOptPass
 ; CHECK-O-NEXT: Running pass: GlobalDCEPass
 ; CHECK-O-NEXT: Running pass: InferFunctionAttrsPass