[llvm] r212307 - InstSimplify: Fix a bug when INT_MIN is in a sdiv

[David Majnemer]

Author: majnemer
Date: Thu Jul  3 19:23:39 2014
New Revision: 212307

URL: http://llvm.org/viewvc/llvm-project?rev=212307&view=rev
Log:
InstSimplify: Fix a bug when INT_MIN is in a sdiv

When INT_MIN is the numerator in a sdiv, we would not properly handle
overflow when calculating the bounds of possible values; abs(INT_MIN) is
not a meaningful number.

Instead, check and handle INT_MIN by reasoning that the largest value is
INT_MIN/-2 and the smallest value is INT_MIN.

This fixes PR20199.

Modified:
    llvm/trunk/lib/Analysis/InstructionSimplify.cpp
    llvm/trunk/test/Transforms/InstSimplify/compare.ll

Modified: llvm/trunk/lib/Analysis/InstructionSimplify.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/InstructionSimplify.cpp?rev=212307&r1=212306&r2=212307&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/InstructionSimplify.cpp (original)
+++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp Thu Jul  3 19:23:39 2014
@@ -1948,9 +1948,15 @@ static Value *SimplifyICmpInst(unsigned
       if (!CI2->isZero())
         Upper = NegOne.udiv(CI2->getValue()) + 1;
     } else if (match(LHS, m_SDiv(m_ConstantInt(CI2), m_Value()))) {
-      // 'sdiv CI2, x' produces [-|CI2|, |CI2|].
-      Upper = CI2->getValue().abs() + 1;
-      Lower = (-Upper) + 1;
+      if (CI2->isMinSignedValue()) {
+        // 'sdiv INT_MIN, x' produces [INT_MIN, INT_MIN / -2].
+        Lower = CI2->getValue();
+        Upper = Lower.lshr(1) + 1;
+      } else {
+        // 'sdiv CI2, x' produces [-|CI2|, |CI2|].
+        Upper = CI2->getValue().abs() + 1;
+        Lower = (-Upper) + 1;
+      }
     } else if (match(LHS, m_SDiv(m_Value(), m_ConstantInt(CI2)))) {
       // 'sdiv x, CI2' produces [INT_MIN / CI2, INT_MAX / CI2].
       APInt IntMin = APInt::getSignedMinValue(Width);

Modified: llvm/trunk/test/Transforms/InstSimplify/compare.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstSimplify/compare.ll?rev=212307&r1=212306&r2=212307&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstSimplify/compare.ll (original)
+++ llvm/trunk/test/Transforms/InstSimplify/compare.ll Thu Jul  3 19:23:39 2014
@@ -902,3 +902,14 @@ define i1 @icmp_ne_const(i32 %a) nounwin
 ; CHECK-LABEL: @icmp_ne_const
 ; CHECK-NEXT: ret i1 true
 }
+
+define i1 @icmp_sdiv_int_min(i32 %a) {
+  %div = sdiv i32 -2147483648, %a
+  %cmp = icmp ne i32 %div, -1073741824
+  ret i1 %cmp
+
+; CHECK-LABEL: @icmp_sdiv_int_min
+; CHECK-NEXT: [[DIV:%.*]] = sdiv i32 -2147483648, %a
+; CHECK-NEXT: [[CMP:%.*]] = icmp ne i32 [[DIV]], -1073741824
+; CHECK-NEXT: ret i1 [[CMP]]
+}