[cfe-commits] r59124 - /cfe/trunk/lib/CodeGen/CGExprScalar.cpp
Chris Lattner
sabre at nondot.org
Wed Nov 12 00:55:54 PST 2008
Author: lattner
Date: Wed Nov 12 02:55:54 2008
New Revision: 59124
URL: http://llvm.org/viewvc/llvm-project?rev=59124&view=rev
Log:
Lower ?: into select when the selected values are cheap and side-effect-free.
This cuts another 200 lines off expr.ll, forming 23 selects.
Modified:
cfe/trunk/lib/CodeGen/CGExprScalar.cpp
Modified: cfe/trunk/lib/CodeGen/CGExprScalar.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/CodeGen/CGExprScalar.cpp?rev=59124&r1=59123&r2=59124&view=diff
==============================================================================
--- cfe/trunk/lib/CodeGen/CGExprScalar.cpp (original)
+++ cfe/trunk/lib/CodeGen/CGExprScalar.cpp Wed Nov 12 02:55:54 2008
@@ -1119,6 +1119,30 @@
// Other Operators
//===----------------------------------------------------------------------===//
+/// isCheapEnoughToEvaluateUnconditionally - Return true if the specified
+/// expression is cheap enough and side-effect-free enough to evaluate
+/// unconditionally instead of conditionally. This is used to convert control
+/// flow into selects in some cases.
+static bool isCheapEnoughToEvaluateUnconditionally(const Expr *E) {
+ if (const ParenExpr *PE = dyn_cast<ParenExpr>(E))
+ return isCheapEnoughToEvaluateUnconditionally(PE->getSubExpr());
+
+ // TODO: Allow anything we can constant fold to an integer or fp constant.
+ if (isa<IntegerLiteral>(E) || isa<CharacterLiteral>(E) ||
+ isa<FloatingLiteral>(E))
+ return true;
+
+ // Non-volatile automatic variables too, to get "cond ? X : Y" where
+ // X and Y are local variables.
+ if (const DeclRefExpr *DRE = dyn_cast<DeclRefExpr>(E))
+ if (const VarDecl *VD = dyn_cast<VarDecl>(DRE->getDecl()))
+ if (VD->hasLocalStorage() && !VD->getType().isVolatileQualified())
+ return true;
+
+ return false;
+}
+
+
Value *ScalarExprEmitter::
VisitConditionalOperator(const ConditionalOperator *E) {
// If the condition constant folds and can be elided, try to avoid emitting
@@ -1136,6 +1160,19 @@
return Visit(Live);
}
+
+ // If this is a really simple expression (like x ? 4 : 5), emit this as a
+ // select instead of as control flow. We can only do this if it is cheap and
+ // safe to
+ if (E->getLHS() && isCheapEnoughToEvaluateUnconditionally(E->getLHS()) &&
+ isCheapEnoughToEvaluateUnconditionally(E->getRHS())) {
+ llvm::Value *CondV = CGF.EvaluateExprAsBool(E->getCond());
+ llvm::Value *LHS = Visit(E->getLHS());
+ llvm::Value *RHS = Visit(E->getRHS());
+ return Builder.CreateSelect(CondV, LHS, RHS, "cond");
+ }
+
+
llvm::BasicBlock *LHSBlock = CGF.createBasicBlock("cond.?");
llvm::BasicBlock *RHSBlock = CGF.createBasicBlock("cond.:");
llvm::BasicBlock *ContBlock = CGF.createBasicBlock("cond.cont");
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