PR9998: ashr exact %x, 31 is not equivalent to sdiv exact %x, -2147483648.

llvm-svn: 132097

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -919,11 +919,11 @@ Instruction *InstCombiner::FoldICmpShrCst(ICmpInst &ICI, BinaryOperator *Shr,
     if (ICI.isSigned() != (Shr->getOpcode() == Instruction::AShr))
       return 0;
     
-    // Otherwise, all lshr and all exact ashr's are equivalent to a udiv/sdiv by
-    // a power of 2.  Since we already have logic to simplify these, transform
-    // to div and then simplify the resultant comparison.
+    // Otherwise, all lshr and most exact ashr's are equivalent to a udiv/sdiv
+    // by a power of 2.  Since we already have logic to simplify these,
+    // transform to div and then simplify the resultant comparison.
     if (Shr->getOpcode() == Instruction::AShr &&
-        !Shr->isExact())
+        (!Shr->isExact() || ShAmtVal == TypeBits - 1))
       return 0;
     
     // Revisit the shift (to delete it).

llvm/test/Transforms/InstCombine/exact.ll

@@ -96,6 +96,22 @@ define i1 @ashr_icmp2(i64 %X) nounwind {
  ret i1 %Z
 }
 
+; PR9998
+; Make sure we don't transform the ashr here into an sdiv
+; CHECK: @pr9998
+; CHECK: = and i32 %V, 1
+; CHECK: %Z = icmp ne
+; CHECK: ret i1 %Z
+define i1 @pr9998(i32 %V) nounwind {
+entry:
+  %W = shl i32 %V, 31
+  %X = ashr exact i32 %W, 31
+  %Y = sext i32 %X to i64
+  %Z = icmp ugt i64 %Y, 7297771788697658747
+  ret i1 %Z
+}
+
+
 ; CHECK: @udiv_icmp1
 ; CHECK: icmp ne i64 %X, 0
 define i1 @udiv_icmp1(i64 %X) nounwind {