[llvm-commits] [llvm-gcc-4.2] r57078 - /llvm-gcc-4.2/trunk/gcc/llvm-convert.cpp

[Chris Lattner]

Author: lattner
Date: Sat Oct  4 17:26:02 2008
New Revision: 57078

URL: http://llvm.org/viewvc/llvm-project?rev=57078&view=rev
Log:
optimize a very very common case for -O0.  We usually get a COND_EXPR with a
comparison as an operand.  Calling 'Emit' on the operand emits the icmp/fcmp,
but then zext's the result out to sizeof(bool).  This compiles:

  if (X < 4) {

into llvm IR like this:

	%1 = icmp sle i32 %0, 3		; <i1> [#uses=1]
	%2 = zext i1 %1 to i8		; <i8> [#uses=1]
	%toBool = icmp ne i8 %2, 0		; <i1> [#uses=1]
	br i1 %toBool, label %bb, label %bb3

Change llvm-convert to avoid inserting the 'toBool' icmp in this case, we
now produce (note that the branch directly uses the icmp):

	%1 = icmp sle i32 %0, 3		; <i1> [#uses=2]
	%2 = zext i1 %1 to i8		; <i8> [#uses=0]
	br i1 %1, label %bb, label %bb2

None of this really matters at -O, but at -O0 with fastisel, this doesn't get
optimized away.  fastisel is still producing ugly code due to the dead 
zext though, stay tuned.


Modified:
    llvm-gcc-4.2/trunk/gcc/llvm-convert.cpp

Modified: llvm-gcc-4.2/trunk/gcc/llvm-convert.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm-gcc-4.2/trunk/gcc/llvm-convert.cpp?rev=57078&r1=57077&r2=57078&view=diff

==============================================================================
--- llvm-gcc-4.2/trunk/gcc/llvm-convert.cpp (original)
+++ llvm-gcc-4.2/trunk/gcc/llvm-convert.cpp Sat Oct  4 17:26:02 2008
@@ -1817,9 +1817,18 @@
   // Emit the conditional expression.
   Value *Cond = Emit(COND_EXPR_COND(exp), 0);
   // If its not already a bool, insert a comparison against zero to make it so.
-  if (Cond->getType() != Type::Int1Ty)
-    Cond = Builder.CreateICmpNE(Cond, Constant::getNullValue(Cond->getType()),
-                                "toBool");
+  if (Cond->getType() != Type::Int1Ty) {
+    // Handle the common case where a boolean expression was evaluated as our
+    // operand, but was then zero extended to a larger integer type.  At -O0, we
+    // don't really want to emit "a = icmp...; b = zext a to i8; c = icmp b, 0"
+    if (ZExtInst *CondI = dyn_cast<ZExtInst>(Cond))
+      if (CondI->getOperand(0)->getType() == Type::Int1Ty)
+        Cond = CondI->getOperand(0);
+    
+    // Otherwise, emit a simple comparison.
+    if (Cond->getType() != Type::Int1Ty)
+      Cond = Builder.CreateIsNotNull(Cond, "toBool");
+  }
 
   tree Then = COND_EXPR_THEN(exp);
   tree Else = COND_EXPR_ELSE(exp);