InstSimplify: Improve handling of ashr/lshr

Summary:
Analyze the range of values produced by ashr/lshr cst, %V when it is
being used in an icmp.

Reviewers: nicholas

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D3774

llvm-svn: 209000

2 files changed, 61 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 4cad81b1398..3684fda854f 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -2001,7 +2001,7 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
 
     // Many binary operators with constant RHS have easy to compute constant
     // range.  Use them to check whether the comparison is a tautology.
-    uint32_t Width = CI->getBitWidth();
+    unsigned Width = CI->getBitWidth();
     APInt Lower = APInt(Width, 0);
     APInt Upper = APInt(Width, 0);
     ConstantInt *CI2;
@@ -2038,6 +2038,13 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
       APInt NegOne = APInt::getAllOnesValue(Width);
       if (CI2->getValue().ult(Width))
         Upper = NegOne.lshr(CI2->getValue()) + 1;
+    } else if (match(LHS, m_LShr(m_ConstantInt(CI2), m_Value()))) {
+      // 'lshr CI2, x' produces [CI2 >> (Width-1), CI2].
+      unsigned ShiftAmount = Width - 1;
+      if (!CI2->isZero() && cast<BinaryOperator>(LHS)->isExact())
+        ShiftAmount = CI2->getValue().countTrailingZeros();
+      Lower = CI2->getValue().lshr(ShiftAmount);
+      Upper = CI2->getValue() + 1;
     } else if (match(LHS, m_AShr(m_Value(), m_ConstantInt(CI2)))) {
       // 'ashr x, CI2' produces [INT_MIN >> CI2, INT_MAX >> CI2].
       APInt IntMin = APInt::getSignedMinValue(Width);
@@ -2046,6 +2053,19 @@ static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
         Lower = IntMin.ashr(CI2->getValue());
         Upper = IntMax.ashr(CI2->getValue()) + 1;
       }
+    } else if (match(LHS, m_AShr(m_ConstantInt(CI2), m_Value()))) {
+      unsigned ShiftAmount = Width - 1;
+      if (!CI2->isZero() && cast<BinaryOperator>(LHS)->isExact())
+        ShiftAmount = CI2->getValue().countTrailingZeros();
+      if (CI2->isNegative()) {
+        // 'ashr CI2, x' produces [CI2, CI2 >> (Width-1)]
+        Lower = CI2->getValue();
+        Upper = CI2->getValue().ashr(ShiftAmount) + 1;
+      } else {
+        // 'ashr CI2, x' produces [CI2 >> (Width-1), CI2]
+        Lower = CI2->getValue().ashr(ShiftAmount);
+        Upper = CI2->getValue() + 1;
+      }
     } else if (match(LHS, m_Or(m_Value(), m_ConstantInt(CI2)))) {
       // 'or x, CI2' produces [CI2, UINT_MAX].
       Lower = CI2->getValue();
diff --git a/llvm/test/Transforms/InstSimplify/compare.ll b/llvm/test/Transforms/InstSimplify/compare.ll
index 0e270bbe0f9..1a62e27dd8c 100644
--- a/llvm/test/Transforms/InstSimplify/compare.ll
+++ b/llvm/test/Transforms/InstSimplify/compare.ll
@@ -826,3 +826,43 @@ define i1 @compare_dividend(i32 %a) {
 ; CHECK-LABEL: @compare_dividend
 ; CHECK-NEXT: ret i1 false
 }
+
+define i1 @lshr_ugt_false(i32 %a) {
+  %shr = lshr i32 1, %a
+  %cmp = icmp ugt i32 %shr, 1
+  ret i1 %cmp
+; CHECK-LABEL: @lshr_ugt_false
+; CHECK-NEXT: ret i1 false
+}
+
+define i1 @exact_lshr_ugt_false(i32 %a) {
+  %shr = lshr exact i32 30, %a
+  %cmp = icmp ult i32 %shr, 15
+  ret i1 %cmp
+; CHECK-LABEL: @exact_lshr_ugt_false
+; CHECK-NEXT: ret i1 false
+}
+
+define i1 @lshr_sgt_false(i32 %a) {
+  %shr = lshr i32 1, %a
+  %cmp = icmp sgt i32 %shr, 1
+  ret i1 %cmp
+; CHECK-LABEL: @lshr_sgt_false
+; CHECK-NEXT: ret i1 false
+}
+
+define i1 @ashr_sgt_false(i32 %a) {
+  %shr = ashr i32 -30, %a
+  %cmp = icmp sgt i32 %shr, -1
+  ret i1 %cmp
+; CHECK-LABEL: @ashr_sgt_false
+; CHECK-NEXT: ret i1 false
+}
+
+define i1 @exact_ashr_sgt_false(i32 %a) {
+  %shr = ashr exact i32 -30, %a
+  %cmp = icmp sgt i32 %shr, -15
+  ret i1 %cmp
+; CHECK-LABEL: @exact_ashr_sgt_false
+; CHECK-NEXT: ret i1 false
+}