DAGCombine: fold "(zext x) == C" into "x == (trunc C)" if the trunc is lossless.

On x86 this allows to fold a load into the cmp, greatly reducing register pressure.
  movzbl	(%rdi), %eax
  cmpl	$47, %eax
->
  cmpb	$47, (%rdi)

This shaves 8k off gcc.o on i386. I'll leave applying the patch in README.txt to Chris :)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130005 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -1916,6 +1916,42 @@ TargetLowering::SimplifySetCC(EVT VT, SDValue N0, SDValue N1,
       // TODO: (ctpop x) == 1 -> x && (x & x-1) == 0 iff ctpop is illegal.
     }
 
+    // (zext x) == C --> x == (trunc C)
+    if (DCI.isBeforeLegalize() && N0->hasOneUse() &&
+        (Cond == ISD::SETEQ || Cond == ISD::SETNE)) {
+      unsigned MinBits = N0.getValueSizeInBits();
+      SDValue PreZExt;
+      if (N0->getOpcode() == ISD::ZERO_EXTEND) {
+        // ZExt
+        MinBits = N0->getOperand(0).getValueSizeInBits();
+        PreZExt = N0->getOperand(0);
+      } else if (N0->getOpcode() == ISD::AND) {
+        // DAGCombine turns costly ZExts into ANDs
+        if (ConstantSDNode *C = dyn_cast<ConstantSDNode>(N0->getOperand(1)))
+          if ((C->getAPIntValue()+1).isPowerOf2()) {
+            MinBits = C->getAPIntValue().countTrailingOnes();
+            PreZExt = N0->getOperand(0);
+          }
+      } else if (LoadSDNode *LN0 = dyn_cast<LoadSDNode>(N0)) {
+        // ZEXTLOAD
+        if (LN0->getExtensionType() == ISD::ZEXTLOAD) {
+          MinBits = LN0->getMemoryVT().getSizeInBits();
+          PreZExt = N0;
+        }
+      }
+
+      // Make sure we're not loosing bits from the constant.
+      if (MinBits < C1.getBitWidth() && MinBits > C1.getActiveBits()) {
+        EVT MinVT = EVT::getIntegerVT(*DAG.getContext(), MinBits);
+        if (isTypeDesirableForOp(ISD::SETCC, MinVT)) {
+          // Will get folded away.
+          SDValue Trunc = DAG.getNode(ISD::TRUNCATE, dl, MinVT, PreZExt);
+          SDValue C = DAG.getConstant(C1.trunc(MinBits), MinVT);
+          return DAG.getSetCC(dl, VT, Trunc, C, Cond);
+        }
+      }
+    }
+
     // If the LHS is '(and load, const)', the RHS is 0,
     // the test is for equality or unsigned, and all 1 bits of the const are
     // in the same partial word, see if we can shorten the load.

lib/Target/README.txt

@@ -2259,34 +2259,6 @@ icmp transform.
 
 //===---------------------------------------------------------------------===//
 
-These functions:
-int foo(int *X) {
-  if ((*X & 255) == 47)
-    bar();
-}
-int foo2(int X) {
-  if ((X & 255) == 47)
-    bar();
-}
-
-codegen to:
-
-  movzbl	(%rdi), %eax
-  cmpl	$47, %eax
-  jne	LBB0_2
-
-and:
-  movzbl	%dil, %eax
-  cmpl	$47, %eax
-  jne	LBB1_2
-
-If a dag combine shrunk the compare to a byte compare, then we'd fold the load
-in the first example, and eliminate the movzbl in the second, saving a register.
-This can be a target independent dag combine that works on ISD::SETCC, it would
-catch this before the legalize ops pass.
-
-//===---------------------------------------------------------------------===//
-
 We should optimize this:
 
   %tmp = load i16* %arrayidx, align 4, !tbaa !0
@@ -2329,8 +2301,7 @@ Index: InstCombine/InstCombineCompares.cpp
    {
 
 
-but we can't do that until the dag combine above is added.  Not having this
-is blocking resolving PR6627.
+Not having this is blocking resolving PR6627.
 
 //===---------------------------------------------------------------------===//
 

test/CodeGen/X86/shrink-compare.ll

@@ -0,0 +1,36 @@
+; RUN: llc < %s -march=x86-64 | FileCheck %s
+
+declare void @bar()
+
+define void @test1(i32* nocapture %X) nounwind {
+entry:
+  %tmp1 = load i32* %X, align 4
+  %and = and i32 %tmp1, 255
+  %cmp = icmp eq i32 %and, 47
+  br i1 %cmp, label %if.then, label %if.end
+
+if.then:
+  tail call void @bar() nounwind
+  br label %if.end
+
+if.end:
+  ret void
+; CHECK: test1:
+; CHECK: cmpb $47, (%rdi)
+}
+
+define void @test2(i32 %X) nounwind {
+entry:
+  %and = and i32 %X, 255
+  %cmp = icmp eq i32 %and, 47
+  br i1 %cmp, label %if.then, label %if.end
+
+if.then:
+  tail call void @bar() nounwind
+  br label %if.end
+
+if.end:
+  ret void
+; CHECK: test2:
+; CHECK: cmpb $47, %dil
+}