# [llvm] r216570 - InstSimplify: Compute comparison ranges for left shift instructions

David Majnemer david.majnemer at gmail.com
Wed Aug 27 11:03:46 PDT 2014

```Author: majnemer
Date: Wed Aug 27 13:03:46 2014
New Revision: 216570

URL: http://llvm.org/viewvc/llvm-project?rev=216570&view=rev
Log:
InstSimplify: Compute comparison ranges for left shift instructions

'shl nuw CI, x' produces [CI, CI << CLZ(CI)]
'shl nsw CI, x' produces [CI << CLO(CI)-1, CI] if CI is negative
'shl nsw CI, x' produces [CI, CI << CLZ(CI)-1] if CI is non-negative

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=216570&r1=216569&r2=216570&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/InstructionSimplify.cpp (original)
+++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp Wed Aug 27 13:03:46 2014
@@ -1993,6 +1993,22 @@ static Value *SimplifyICmpInst(unsigned
Upper = Upper + 1;
assert(Upper != Lower && "Upper part of range has wrapped!");
}
+    } else if (match(LHS, m_NUWShl(m_ConstantInt(CI2), m_Value()))) {
+      // 'shl nuw CI2, x' produces [CI2, CI2 << CLZ(CI2)]
+      Lower = CI2->getValue();
+      Upper = Lower.shl(Lower.countLeadingZeros()) + 1;
+    } else if (match(LHS, m_NSWShl(m_ConstantInt(CI2), m_Value()))) {
+      if (CI2->isNegative()) {
+        // 'shl nsw CI2, x' produces [CI2 << CLO(CI2)-1, CI2]
+        unsigned ShiftAmount = CI2->getValue().countLeadingOnes() - 1;
+        Lower = CI2->getValue().shl(ShiftAmount);
+        Upper = CI2->getValue() + 1;
+      } else {
+        // 'shl nsw CI2, x' produces [CI2, CI2 << CLZ(CI2)-1]
+        unsigned ShiftAmount = CI2->getValue().countLeadingZeros() - 1;
+        Lower = CI2->getValue();
+        Upper = CI2->getValue().shl(ShiftAmount) + 1;
+      }
} else if (match(LHS, m_LShr(m_Value(), m_ConstantInt(CI2)))) {
// 'lshr x, CI2' produces [0, UINT_MAX >> CI2].
APInt NegOne = APInt::getAllOnesValue(Width);

Modified: llvm/trunk/test/Transforms/InstSimplify/compare.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstSimplify/compare.ll?rev=216570&r1=216569&r2=216570&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstSimplify/compare.ll (original)
+++ llvm/trunk/test/Transforms/InstSimplify/compare.ll Wed Aug 27 13:03:46 2014
@@ -982,3 +982,30 @@ define i1 @icmp_known_bits(i4 %x, i4 %y)
; CHECK-LABEL: @icmp_known_bits
; CHECK-NEXT: ret i1 false
}
+
+define i1 @icmp_shl_nuw_1(i64 %a) {
+ %shl = shl nuw i64 1, %a
+ %cmp = icmp ne i64 %shl, 0
+ ret i1 %cmp
+
+; CHECK-LABEL: @icmp_shl_nuw_1
+; CHECK-NEXT: ret i1 true
+}
+
+define i1 @icmp_shl_nsw_neg1(i64 %a) {
+ %shl = shl nsw i64 -1, %a
+ %cmp = icmp sge i64 %shl, 3
+ ret i1 %cmp
+
+; CHECK-LABEL: @icmp_shl_nsw_neg1
+; CHECK-NEXT: ret i1 false
+}
+
+define i1 @icmp_shl_nsw_1(i64 %a) {
+ %shl = shl nsw i64 1, %a
+ %cmp = icmp sge i64 %shl, 0
+ ret i1 %cmp
+
+; CHECK-LABEL: @icmp_shl_nsw_1
+; CHECK-NEXT: ret i1 true
+}

```