[llvm] r219713 - InstCombine: Don't miscompile X % ((Pow2 << A) >>u B)

[David Majnemer]

Author: majnemer
Date: Tue Oct 14 15:28:40 2014
New Revision: 219713

URL: http://llvm.org/viewvc/llvm-project?rev=219713&view=rev
Log:
InstCombine: Don't miscompile X % ((Pow2 << A) >>u B)

We assumed that A must be greater than B because the right hand side of
a remainder operator must be nonzero.

However, it is possible for A to be less than B if Pow2 is a power of
two greater than 1.

Take for example:
i32 %A = 0
i32 %B = 31
i32 Pow2 = 2147483648

((Pow2 << 0) >>u 31) is non-zero but A is less than B.

This fixes PR21274.

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

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp?rev=219713&r1=219712&r2=219713&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp Tue Oct 14 15:28:40 2014
@@ -36,14 +36,11 @@ static Value *simplifyValueKnownNonZero(
 
   // ((1 << A) >>u B) --> (1 << (A-B))
   // Because V cannot be zero, we know that B is less than A.
-  Value *A = nullptr, *B = nullptr, *PowerOf2 = nullptr;
-  if (match(V, m_LShr(m_OneUse(m_Shl(m_Value(PowerOf2), m_Value(A))),
-                      m_Value(B))) &&
-      // The "1" can be any value known to be a power of 2.
-      isKnownToBeAPowerOfTwo(PowerOf2, false, 0, IC.getAssumptionTracker(),
-                             CxtI, IC.getDominatorTree())) {
+  Value *A = nullptr, *B = nullptr, *One = nullptr;
+  if (match(V, m_LShr(m_OneUse(m_Shl(m_Value(One), m_Value(A))), m_Value(B))) &&
+      match(One, m_One())) {
     A = IC.Builder->CreateSub(A, B);
-    return IC.Builder->CreateShl(PowerOf2, A);
+    return IC.Builder->CreateShl(One, A);
   }
 
   // (PowerOfTwo >>u B) --> isExact since shifting out the result would make it

Modified: llvm/trunk/test/Transforms/InstCombine/div.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/div.ll?rev=219713&r1=219712&r2=219713&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/div.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/div.ll Tue Oct 14 15:28:40 2014
@@ -265,7 +265,7 @@ define i32 @test30(i32 %a) {
 ; CHECK-NEXT: ret i32 %a
 }
 
-define <2 x i32> @test31(<2 x i32> %x) nounwind {
+define <2 x i32> @test31(<2 x i32> %x) {
   %shr = lshr <2 x i32> %x, <i32 31, i32 31>
   %div = udiv <2 x i32> %shr, <i32 2147483647, i32 2147483647>
   ret <2 x i32> %div
@@ -274,3 +274,15 @@ define <2 x i32> @test31(<2 x i32> %x) n
 ; CHECK-NEXT: udiv <2 x i32> %[[shr]], <i32 2147483647, i32 2147483647>
 ; CHECK-NEXT: ret <2 x i32>
 }
+
+define i32 @test32(i32 %a, i32 %b) {
+  %shl = shl i32 2, %b
+  %div = lshr i32 %shl, 2
+  %div2 = udiv i32 %a, %div
+  ret i32 %div2
+; CHECK-LABEL: @test32(
+; CHECK-NEXT: %[[shl:.*]] = shl i32 2, %b
+; CHECK-NEXT: %[[shr:.*]] = lshr i32 %[[shl]], 2
+; CHECK-NEXT: %[[div:.*]] = udiv i32 %a, %[[shr]]
+; CHECK-NEXT: ret i32
+}