[llvm-commits] [llvm] r131887 - in /llvm/trunk: lib/Transforms/InstCombine/InstCombineMulDivRem.cpp test/Transforms/InstCombine/shift.ll

[Chris Lattner]

Author: lattner
Date: Sun May 22 19:21:50 2011
New Revision: 131887

URL: http://llvm.org/viewvc/llvm-project?rev=131887&view=rev
Log:
Transform any logical shift of a power of two into an exact/NUW shift when
in a known-non-zero context.

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
    llvm/trunk/test/Transforms/InstCombine/shift.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp?rev=131887&r1=131886&r2=131887&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp Sun May 22 19:21:50 2011
@@ -29,6 +29,23 @@
   // code.
   if (!V->hasOneUse()) return 0;
   
+  
+  // (PowerOfTwo >>u B) --> isExact since shifting out the result would make it
+  // inexact.  Similarly for <<.
+  if (BinaryOperator *I = dyn_cast<BinaryOperator>(V))
+    if (I->isLogicalShift() &&
+        isPowerOfTwo(I->getOperand(0), IC.getTargetData())) {
+      if (I->getOpcode() == Instruction::LShr && !I->isExact()) {
+        I->setIsExact();
+        return I;
+      }
+      
+      if (I->getOpcode() == Instruction::Shl && !I->hasNoUnsignedWrap()) {
+        I->setHasNoUnsignedWrap();
+        return I;
+      }
+    }
+      
   // ((1 << A) >>u B) --> (1 << (A-B))
   // Because V cannot be zero, we know that B is less than A.
   Value *A = 0, *B = 0, *PowerOf2 = 0;

Modified: llvm/trunk/test/Transforms/InstCombine/shift.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/shift.ll?rev=131887&r1=131886&r2=131887&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/shift.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/shift.ll Sun May 22 19:21:50 2011
@@ -506,3 +506,24 @@
 ; CHECK-NEXT: shl i32 8, %b
 ; CHECK-NEXT: ret i32
 }
+
+define i32 @test42(i32 %a, i32 %b) nounwind {
+  %div = lshr i32 4096, %b    ; must be exact otherwise we'd divide by zero
+  %div2 = udiv i32 %a, %div
+  ret i32 %div2
+; CHECK: @test42
+; CHECK-NEXT: lshr exact i32 4096, %b
+}
+
+define i32 @test43(i32 %a, i32 %b) nounwind {
+  %div = shl i32 4096, %b    ; must be exact otherwise we'd divide by zero
+  %div2 = udiv i32 %a, %div
+  ret i32 %div2
+; CHECK: @test43
+; CHECK-NEXT: add i32 %b, 12
+; CHECK-NEXT: lshr
+; CHECK-NEXT: ret
+}
+
+
+