[llvm-commits] [llvm] r147255 - in /llvm/trunk: lib/Analysis/ValueTracking.cpp test/Transforms/InstCombine/intrinsics.ll test/Transforms/InstCombine/sub-xor.ll

[Benjamin Kramer]

Author: d0k
Date: Sat Dec 24 11:31:46 2011
New Revision: 147255

URL: http://llvm.org/viewvc/llvm-project?rev=147255&view=rev
Log:
ComputeMaskedBits: Make knownzero computation more aggressive for ctlz with undef zero.

unsigned foo(unsigned x) { return 31 - __builtin_clz(x); }
now compiles into a single "bsrl" instruction on x86.

Modified:
    llvm/trunk/lib/Analysis/ValueTracking.cpp
    llvm/trunk/test/Transforms/InstCombine/intrinsics.ll
    llvm/trunk/test/Transforms/InstCombine/sub-xor.ll

Modified: llvm/trunk/lib/Analysis/ValueTracking.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ValueTracking.cpp?rev=147255&r1=147254&r2=147255&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ValueTracking.cpp (original)
+++ llvm/trunk/lib/Analysis/ValueTracking.cpp Sat Dec 24 11:31:46 2011
@@ -714,10 +714,17 @@
     if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
       switch (II->getIntrinsicID()) {
       default: break;
-      case Intrinsic::ctpop:
       case Intrinsic::ctlz:
       case Intrinsic::cttz: {
         unsigned LowBits = Log2_32(BitWidth)+1;
+        // If this call is undefined for 0, the result will be less than 2^n.
+        if (II->getArgOperand(1) == ConstantInt::getTrue(II->getContext()))
+          LowBits -= 1;
+        KnownZero = APInt::getHighBitsSet(BitWidth, BitWidth - LowBits);
+        break;
+      }
+      case Intrinsic::ctpop: {
+        unsigned LowBits = Log2_32(BitWidth)+1;
         KnownZero = APInt::getHighBitsSet(BitWidth, BitWidth - LowBits);
         break;
       }

Modified: llvm/trunk/test/Transforms/InstCombine/intrinsics.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/intrinsics.ll?rev=147255&r1=147254&r2=147255&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/intrinsics.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/intrinsics.ll Sat Dec 24 11:31:46 2011
@@ -181,10 +181,10 @@
 
 define void @cmp.simplify(i32 %a, i32 %b, i1* %c) {
 entry:
-  %lz = tail call i32 @llvm.ctlz.i32(i32 %a, i1 true) nounwind readnone
+  %lz = tail call i32 @llvm.ctlz.i32(i32 %a, i1 false) nounwind readnone
   %lz.cmp = icmp eq i32 %lz, 32
   store volatile i1 %lz.cmp, i1* %c
-  %tz = tail call i32 @llvm.cttz.i32(i32 %a, i1 true) nounwind readnone
+  %tz = tail call i32 @llvm.cttz.i32(i32 %a, i1 false) nounwind readnone
   %tz.cmp = icmp ne i32 %tz, 32
   store volatile i1 %tz.cmp, i1* %c
   %pop = tail call i32 @llvm.ctpop.i32(i32 %b) nounwind readnone
@@ -203,7 +203,7 @@
 
 
 define i32 @cttz_simplify1(i32 %x) nounwind readnone ssp {
-  %tmp1 = tail call i32 @llvm.ctlz.i32(i32 %x, i1 true)    ; <i32> [#uses=1]
+  %tmp1 = tail call i32 @llvm.ctlz.i32(i32 %x, i1 false)    ; <i32> [#uses=1]
   %shr3 = lshr i32 %tmp1, 5                       ; <i32> [#uses=1]
   ret i32 %shr3
   

Modified: llvm/trunk/test/Transforms/InstCombine/sub-xor.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/sub-xor.ll?rev=147255&r1=147254&r2=147255&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/sub-xor.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/sub-xor.ll Sat Dec 24 11:31:46 2011
@@ -10,3 +10,16 @@
 ; CHECK-NEXT: xor i32 %and, 63
 ; CHECK-NEXT: ret
 }
+
+declare i32 @llvm.ctlz.i32(i32, i1) nounwind readnone
+
+define i32 @test2(i32 %x) nounwind {
+  %count = tail call i32 @llvm.ctlz.i32(i32 %x, i1 true) nounwind readnone
+  %sub = sub i32 31, %count
+  ret i32 %sub
+
+; CHECK: @test2
+; CHECK-NEXT: ctlz
+; CHECK-NEXT: xor i32 %count, 31
+; CHECK-NEXT: ret
+}