Fix PR2697 by rewriting the '(X / pos) op neg' logic. This also changes

a couple other cases for clarity, but shouldn't affect correctness.

Patch by Eli Friedman!



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@57387 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 23 insertions, 10 deletions

diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index 60da031..162e1bf 100644
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -5784,6 +5784,11 @@ Instruction *InstCombiner::FoldICmpDivCst(ICmpInst &ICI, BinaryOperator *DivI,
     return 0;
   if (DivRHS->isZero())
     return 0; // The ProdOV computation fails on divide by zero.
+  if (DivIsSigned && DivRHS->isAllOnesValue())
+    return 0; // The overflow computation also screws up here
+  if (DivRHS->isOne())
+    return 0; // Not worth bothering, and eliminates some funny cases
+              // with INT_MIN.
 
   // Compute Prod = CI * DivRHS. We are essentially solving an equation
   // of form X/C1=C2. We solve for X by multiplying C1 (DivRHS) and 
@@ -5810,7 +5815,6 @@ Instruction *InstCombiner::FoldICmpDivCst(ICmpInst &ICI, BinaryOperator *DivI,
   int LoOverflow = 0, HiOverflow = 0;
   ConstantInt *LoBound = 0, *HiBound = 0;
   
-  
   if (!DivIsSigned) {  // udiv
     // e.g. X/5 op 3  --> [15, 20)
     LoBound = Prod;
@@ -5829,11 +5833,13 @@ Instruction *InstCombiner::FoldICmpDivCst(ICmpInst &ICI, BinaryOperator *DivI,
         HiOverflow = AddWithOverflow(HiBound, Prod, DivRHS, true);
     } else {                       // (X / pos) op neg
       // e.g. X/5 op -3  --> [-15-4, -15+1) --> [-19, -14)
-      Constant *DivRHSH = ConstantExpr::getNeg(SubOne(DivRHS));
-      LoOverflow = AddWithOverflow(LoBound, Prod,
-                                   cast<ConstantInt>(DivRHSH), true) ? -1 : 0;
       HiBound = AddOne(Prod);
-      HiOverflow = ProdOV ? -1 : 0;
+      LoOverflow = HiOverflow = ProdOV ? -1 : 0;
+      if (!LoOverflow) {
+        ConstantInt* DivNeg = cast<ConstantInt>(ConstantExpr::getNeg(DivRHS));
+        LoOverflow = AddWithOverflow(LoBound, HiBound, DivNeg,
+                                     true) ? -1 : 0;
+       }
     }
   } else if (DivRHS->getValue().isNegative()) { // Divisor is < 0.
     if (CmpRHSV == 0) {       // (X / neg) op 0
@@ -5846,14 +5852,13 @@ Instruction *InstCombiner::FoldICmpDivCst(ICmpInst &ICI, BinaryOperator *DivI,
       }
     } else if (CmpRHSV.isStrictlyPositive()) {   // (X / neg) op pos
       // e.g. X/-5 op 3  --> [-19, -14)
+      HiBound = AddOne(Prod);
       HiOverflow = LoOverflow = ProdOV ? -1 : 0;
       if (!LoOverflow)
-        LoOverflow = AddWithOverflow(LoBound, Prod, AddOne(DivRHS), true) ?-1:0;
-      HiBound = AddOne(Prod);
+        LoOverflow = AddWithOverflow(LoBound, HiBound, DivRHS, true) ? -1 : 0;
     } else {                       // (X / neg) op neg
-      // e.g. X/-5 op -3  --> [15, 20)
-      LoBound = Prod;
-      LoOverflow = HiOverflow = ProdOV ? 1 : 0;
+      LoBound = Prod;       // e.g. X/-5 op -3  --> [15, 20)
+      LoOverflow = HiOverflow = ProdOV;
       if (!HiOverflow)
         HiOverflow = SubWithOverflow(HiBound, Prod, DivRHS, true);
     }
diff --git a/test/Transforms/InstCombine/2008-10-11-DivCompareFold.ll b/test/Transforms/InstCombine/2008-10-11-DivCompareFold.ll
new file mode 100644
index 0000000..dfe9c4a
--- /dev/null
+++ b/test/Transforms/InstCombine/2008-10-11-DivCompareFold.ll
@@ -0,0 +1,8 @@
+; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep {ret i1 false}
+; PR2697
+
+define i1 @x(i32 %x) nounwind {
+	%div = sdiv i32 %x, 65536		; <i32> [#uses=1]
+	%cmp = icmp slt i32 %div, -65536
+	ret i1 %cmp
+}