Conditionally eliminate library calls where the result value is not used

Summary:
This pass shrink-wraps a condition to some library calls where the call
result is not used. For example:
   sqrt(val);
 is transformed to
   if (val < 0)
     sqrt(val);
Even if the result of library call is not being used, the compiler cannot
safely delete the call because the function can set errno on error
conditions.
Note in many functions, the error condition solely depends on the incoming
parameter. In this optimization, we can generate the condition can lead to
the errno to shrink-wrap the call. Since the chances of hitting the error
condition is low, the runtime call is effectively eliminated.

These partially dead calls are usually results of C++ abstraction penalty
exposed by inlining. This optimization hits 108 times in 19 C/C++ programs
in SPEC2006.

Reviewers: hfinkel, mehdi_amini, davidxl

Subscribers: modocache, mgorny, mehdi_amini, xur, llvm-commits, beanz

Differential Revision: https://reviews.llvm.org/D24414

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@284542 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/InitializePasses.h

@@ -172,6 +172,7 @@ void initializeLazyBranchProbabilityInfoPassPass(PassRegistry&);
 void initializeLazyBlockFrequencyInfoPassPass(PassRegistry&);
 void initializeLazyValueInfoWrapperPassPass(PassRegistry&);
 void initializeLegalizerPass(PassRegistry&);
+void initializeLibCallsShrinkWrapLegacyPassPass(PassRegistry&);
 void initializeLintPass(PassRegistry&);
 void initializeLiveDebugValuesPass(PassRegistry&);
 void initializeLiveDebugVariablesPass(PassRegistry&);

include/llvm/LinkAllPasses.h

@@ -78,6 +78,7 @@ namespace {
       (void) llvm::createCFLAndersAAWrapperPass();
       (void) llvm::createCFLSteensAAWrapperPass();
       (void) llvm::createStructurizeCFGPass();
+      (void) llvm::createLibCallsShrinkWrapPass();
       (void) llvm::createConstantMergePass();
       (void) llvm::createConstantPropagationPass();
       (void) llvm::createCostModelAnalysisPass();

include/llvm/Transforms/Scalar.h

@@ -519,6 +519,12 @@ FunctionPass *createLoopDataPrefetchPass();
 ///===---------------------------------------------------------------------===//
 ModulePass *createNameAnonGlobalPass();
 
+//===----------------------------------------------------------------------===//
+//
+// LibCallsShrinkWrap - Shrink-wraps a call to function if the result is not
+// used.
+//
+FunctionPass *createLibCallsShrinkWrapPass();
 } // End llvm namespace
 
 #endif

include/llvm/Transforms/Utils/LibCallsShrinkWrap.h

@@ -0,0 +1,27 @@
+//===- LibCallsShrinkWrap.h - Shrink Wrap Library Calls -------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TRANSFORMS_UTILS_LIBCALLSSHRINKWRAP_H
+#define LLVM_TRANSFORMS_UTILS_LIBCALLSSHRINKWRAP_H
+
+#include "llvm/IR/PassManager.h"
+
+namespace llvm {
+
+class LibCallsShrinkWrapPass : public PassInfoMixin<LibCallsShrinkWrapPass> {
+public:
+  static StringRef name() { return "LibCallsShrinkWrapPass"; }
+
+  PreservedAnalyses run(Function &F, FunctionAnalysisManager &FAM);
+};
+} // end namespace llvm
+
+#endif // LLVM_TRANSFORMS_UTILS_LIBCALLSSHRINKWRAP_H

lib/Passes/PassBuilder.cpp

@@ -121,6 +121,7 @@
 #include "llvm/Transforms/Utils/AddDiscriminators.h"
 #include "llvm/Transforms/Utils/BreakCriticalEdges.h"
 #include "llvm/Transforms/Utils/LCSSA.h"
+#include "llvm/Transforms/Utils/LibCallsShrinkWrap.h"
 #include "llvm/Transforms/Utils/LoopSimplify.h"
 #include "llvm/Transforms/Utils/LowerInvoke.h"
 #include "llvm/Transforms/Utils/Mem2Reg.h"

lib/Passes/PassRegistry.def

@@ -149,6 +149,7 @@ FUNCTION_PASS("instsimplify", InstSimplifierPass())
 FUNCTION_PASS("invalidate<all>", InvalidateAllAnalysesPass())
 FUNCTION_PASS("float2int", Float2IntPass())
 FUNCTION_PASS("no-op-function", NoOpFunctionPass())
+FUNCTION_PASS("libcalls-shrinkwrap", LibCallsShrinkWrapPass())
 FUNCTION_PASS("loweratomic", LowerAtomicPass())
 FUNCTION_PASS("lower-expect", LowerExpectIntrinsicPass())
 FUNCTION_PASS("lower-guard-intrinsic", LowerGuardIntrinsicPass())

lib/Transforms/IPO/PassManagerBuilder.cpp

@@ -145,6 +145,11 @@ static cl::opt<bool> EnableGVNHoist(
     "enable-gvn-hoist", cl::init(true), cl::Hidden,
     cl::desc("Enable the GVN hoisting pass (default = on)"));
 
+static cl::opt<bool>
+    DisableLibCallsShrinkWrap("disable-libcalls-shrinkwrap", cl::init(false),
+                              cl::Hidden,
+                              cl::desc("Disable shrink-wrap library calls"));
+
 PassManagerBuilder::PassManagerBuilder() {
     OptLevel = 2;
     SizeLevel = 0;
@@ -297,6 +302,8 @@ void PassManagerBuilder::addFunctionSimplificationPasses(
   MPM.add(createCFGSimplificationPass());     // Merge & remove BBs
   // Combine silly seq's
   addInstructionCombiningPass(MPM);
+  if (SizeLevel == 0 && !DisableLibCallsShrinkWrap)
+    MPM.add(createLibCallsShrinkWrapPass());
   addExtensionsToPM(EP_Peephole, MPM);
 
   MPM.add(createTailCallEliminationPass()); // Eliminate tail calls

lib/Transforms/Utils/CMakeLists.txt

@@ -20,6 +20,7 @@ add_llvm_library(LLVMTransformUtils
   InstructionNamer.cpp
   IntegerDivision.cpp
   LCSSA.cpp
+  LibCallsShrinkWrap.cpp
   Local.cpp
   LoopSimplify.cpp
   LoopUnroll.cpp