[llvm] r212981 - InstSimplify: Correct sdiv x / -1

[David Majnemer]

Author: majnemer
Date: Mon Jul 14 15:38:45 2014
New Revision: 212981

URL: http://llvm.org/viewvc/llvm-project?rev=212981&view=rev
Log:
InstSimplify: Correct sdiv x / -1

Determining the bounds of x/ -1 would start off with us dividing it by
INT_MIN.  Suffice to say, this would not work very well.

Instead, handle it upfront by checking for -1 and mapping it to the
range: [INT_MIN + 1, INT_MAX.  This means that the result of our
division can be any value other than INT_MIN.

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=212981&r1=212980&r2=212981&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/InstructionSimplify.cpp (original)
+++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp Mon Jul 14 15:38:45 2014
@@ -1958,20 +1958,22 @@ static Value *SimplifyICmpInst(unsigned
         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);
       APInt IntMax = APInt::getSignedMaxValue(Width);
-      APInt Val = CI2->getValue().abs();
-      if (!Val.isMinValue()) {
+      APInt Val = CI2->getValue();
+      if (Val.isAllOnesValue()) {
+        // 'sdiv x, -1' produces [INT_MIN + 1, INT_MAX]
+        //    where CI2 != -1 and CI2 != 0 and CI2 != 1
+        Lower = IntMin + 1;
+        Upper = IntMax + 1;
+      } else if (Val.countLeadingZeros() < Width - 1) {
+        // 'sdiv x, CI2' produces [INT_MIN / CI2, INT_MAX / CI2]
+        //    where CI2 != -1 and CI2 != 0 and CI2 != 1
         Lower = IntMin.sdiv(Val);
-        APInt Rem;
-        APInt::sdivrem(IntMax, Val, Upper, Rem);
-        // We may need to round the result of the INT_MAX / CI2 calculation up
-        // if we see that the division was not exact.
-        if (Rem.isMinValue())
-          Upper = Upper + 1;
-        else
-          Upper = Upper + 2;
+        Upper = IntMax.sdiv(Val);
+        if (Lower.sgt(Upper))
+          std::swap(Lower, Upper);
+        Upper = Upper + 1;
         assert(Upper != Lower && "Upper part of range has wrapped!");
       }
     } else if (match(LHS, m_LShr(m_Value(), m_ConstantInt(CI2)))) {

Modified: llvm/trunk/test/Transforms/InstSimplify/compare.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstSimplify/compare.ll?rev=212981&r1=212980&r2=212981&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstSimplify/compare.ll (original)
+++ llvm/trunk/test/Transforms/InstSimplify/compare.ll Mon Jul 14 15:38:45 2014
@@ -924,3 +924,14 @@ define i1 @icmp_sdiv_pr20288(i64 %a) {
 ; CHECK-NEXT: [[CMP:%.*]] = icmp ne i64 [[DIV]], 1073741824
 ; CHECK-NEXT: ret i1 [[CMP]]
 }
+
+define i1 @icmp_sdiv_neg1(i64 %a) {
+ %div = sdiv i64 %call, -1
+ %cmp = icmp ne i64 %div, 1073741824
+ ret i1 %cmp
+
+; CHECK-LABEL: @icmp_sdiv_neg1
+; CHECK-NEXT: [[DIV:%.*]] = sdiv i64 %a, -1
+; CHECK-NEXT: [[CMP:%.*]] = icmp ne i64 [[DIV]], 1073741824
+; CHECK-NEXT: ret i1 [[CMP]]
+}