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add __builtin_unpredictable and convert to metadata

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This patch depends on r246688 (D12341).

The goal is to make LLVM generate different code for these functions for a target that
has cheap branches (see PR23827 for more details):

int foo();

int normal(int x, int y, int z) {
   if (x != 0 && y != 0) return foo();
   return 1;
}

int crazy(int x, int y) {
   if (__builtin_unpredictable(x != 0 && y != 0)) return foo();
   return 1;
}

Differential Revision: http://reviews.llvm.org/D12458

llvm-svn: 246699
This commit is contained in:
Sanjay Patel
2015-09-02 20:01:30 +00:00
parent d428e203a0
commit a24296b459
6 changed files with 93 additions and 3 deletions
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19
clang/lib/CodeGen/CodeGenFunction.cpp
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@@ -24,6 +24,7 @@
#include "clang/AST/Decl.h"
#include "clang/AST/DeclCXX.h"
#include "clang/AST/StmtCXX.h"
#include "clang/Basic/Builtins.h"
#include "clang/Basic/TargetInfo.h"
#include "clang/CodeGen/CGFunctionInfo.h"
#include "clang/Frontend/CodeGenOptions.h"
@@ -1203,6 +1204,22 @@ void CodeGenFunction::EmitBranchOnBoolExpr(const Expr *Cond,
return;
}
// If the branch has a condition wrapped by __builtin_unpredictable,
// create metadata that specifies that the branch is unpredictable.
// Don't bother if not optimizing because that metadata would not be used.
llvm::MDNode *Unpredictable = nullptr;
if (CGM.getCodeGenOpts().OptimizationLevel != 0) {
if (const CallExpr *Call = dyn_cast<CallExpr>(Cond)) {
const Decl *TargetDecl = Call->getCalleeDecl();
if (const FunctionDecl *FD = dyn_cast_or_null<FunctionDecl>(TargetDecl)) {
if (FD->getBuiltinID() == Builtin::BI__builtin_unpredictable) {
llvm::MDBuilder MDHelper(getLLVMContext());
Unpredictable = MDHelper.createUnpredictable();
}
}
}
}
// Create branch weights based on the number of times we get here and the
// number of times the condition should be true.
uint64_t CurrentCount = std::max(getCurrentProfileCount(), TrueCount);
@@ -1215,7 +1232,7 @@ void CodeGenFunction::EmitBranchOnBoolExpr(const Expr *Cond,
ApplyDebugLocation DL(*this, Cond);
CondV = EvaluateExprAsBool(Cond);
}
Builder.CreateCondBr(CondV, TrueBlock, FalseBlock, Weights);
Builder.CreateCondBr(CondV, TrueBlock, FalseBlock, Weights, Unpredictable);
}
/// ErrorUnsupported - Print out an error that codegen doesn't support the