Fix PR6165. The bug was that LHSKnownZero was being and'd with DemandedMask

when it should have been and'd with LowBits.  Fix that and while there beef
up the logic in the case of a negative LHS.

llvm-svn: 94745

2 files changed, 30 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index 74a1b6803d4..ec1ed664961 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -681,10 +681,19 @@ Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
                                  LHSKnownZero, LHSKnownOne, Depth+1))
           return I;
 
+        // The low bits of LHS are unchanged by the srem.
+        KnownZero |= LHSKnownZero & LowBits;
+        KnownOne |= LHSKnownOne & LowBits;
+
+        // If LHS is non-negative or has all low bits zero, then the upper bits
+        // are all zero.
         if (LHSKnownZero[BitWidth-1] || ((LHSKnownZero & LowBits) == LowBits))
-          LHSKnownZero |= ~LowBits;
+          KnownZero |= ~LowBits;
 
-        KnownZero |= LHSKnownZero & DemandedMask;
+        // If LHS is negative and not all low bits are zero, then the upper bits
+        // are all one.
+        if (LHSKnownOne[BitWidth-1] && ((LHSKnownOne & LowBits) != 0))
+          KnownOne |= ~LowBits;
 
         assert(!(KnownZero & KnownOne) && "Bits known to be one AND zero?"); 
       }
diff --git a/llvm/test/Transforms/InstCombine/2010-01-28-NegativeSRem.ll b/llvm/test/Transforms/InstCombine/2010-01-28-NegativeSRem.ll
new file mode 100644
index 00000000000..4ab9bf0c3f5
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/2010-01-28-NegativeSRem.ll
@@ -0,0 +1,19 @@
+; RUN: opt < %s -instcombine -S | FileCheck %s
+; PR6165
+
+define i32 @f() {
+entry:
+  br label %BB1
+
+BB1:                                              ; preds = %BB1, %entry
+; CHECK: BB1:
+  %x = phi i32 [ -29, %entry ], [ 0, %BB1 ]       ; <i32> [#uses=2]
+  %rem = srem i32 %x, 2                           ; <i32> [#uses=1]
+  %t = icmp eq i32 %rem, -1                       ; <i1> [#uses=1]
+  br i1 %t, label %BB2, label %BB1
+; CHECK-NOT: br i1 false
+
+BB2:                                              ; preds = %BB1
+; CHECK: BB2:
+  ret i32 %x
+}