[llvm] r252629 - [ValueTracking] Recognize that and(x, add (x, -1)) clears the low bit
Philip Reames via llvm-commits
llvm-commits at lists.llvm.org
Tue Nov 10 10:46:14 PST 2015
Author: reames
Date: Tue Nov 10 12:46:14 2015
New Revision: 252629
URL: http://llvm.org/viewvc/llvm-project?rev=252629&view=rev
Log:
[ValueTracking] Recognize that and(x, add (x, -1)) clears the low bit
This is a cleaned up version of a patch by John Regehr with permission. Originally found via the souper tool.
If we add an odd number to x, then bitwise-and the result with x, we know that the low bit of the result must be zero. Either it was zero in x originally, or the add cleared it in the temporary value. As a result, one of the two values anded together must have the bit cleared.
Differential Revision: http://reviews.llvm.org/D14315
Added:
llvm/trunk/test/Transforms/InstSimplify/add-mask.ll
Modified:
llvm/trunk/lib/Analysis/ValueTracking.cpp
Modified: llvm/trunk/lib/Analysis/ValueTracking.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ValueTracking.cpp?rev=252629&r1=252628&r2=252629&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ValueTracking.cpp (original)
+++ llvm/trunk/lib/Analysis/ValueTracking.cpp Tue Nov 10 12:46:14 2015
@@ -1077,6 +1077,22 @@ static void computeKnownBitsFromOperator
KnownOne &= KnownOne2;
// Output known-0 are known to be clear if zero in either the LHS | RHS.
KnownZero |= KnownZero2;
+
+ // and(x, add (x, -1)) is a common idiom that always clears the low bit;
+ // here we handle the more general case of adding any odd number by
+ // matching the form add(x, add(x, y)) where y is odd.
+ // TODO: This could be generalized to clearing any bit set in y where the
+ // following bit is known to be unset in y.
+ Value *Y = nullptr;
+ if (match(I->getOperand(0), m_Add(m_Specific(I->getOperand(1)),
+ m_Value(Y))) ||
+ match(I->getOperand(1), m_Add(m_Specific(I->getOperand(0)),
+ m_Value(Y)))) {
+ APInt KnownZero3(BitWidth, 0), KnownOne3(BitWidth, 0);
+ computeKnownBits(Y, KnownZero3, KnownOne3, DL, Depth + 1, Q);
+ if (KnownOne3.countTrailingOnes() > 0)
+ KnownZero |= APInt::getLowBitsSet(BitWidth, 1);
+ }
break;
}
case Instruction::Or: {
Added: llvm/trunk/test/Transforms/InstSimplify/add-mask.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstSimplify/add-mask.ll?rev=252629&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/InstSimplify/add-mask.ll (added)
+++ llvm/trunk/test/Transforms/InstSimplify/add-mask.ll Tue Nov 10 12:46:14 2015
@@ -0,0 +1,65 @@
+; RUN: opt -S -instsimplify < %s | FileCheck %s
+
+define i1 @test(i32 %a) {
+; CHECK-LABEL: @test
+; CHECK: ret i1 false
+ %rhs = add i32 %a, -1
+ %and = and i32 %a, %rhs
+ %res = icmp eq i32 %and, 1
+ ret i1 %res
+}
+
+define i1 @test2(i32 %a) {
+; CHECK-LABEL: @test2
+; CHECK: ret i1 false
+ %rhs = add i32 %a, 1
+ %and = and i32 %a, %rhs
+ %res = icmp eq i32 %and, 1
+ ret i1 %res
+}
+
+define i1 @test3(i32 %a) {
+; CHECK-LABEL: @test3
+; CHECK: ret i1 false
+ %rhs = add i32 %a, 7
+ %and = and i32 %a, %rhs
+ %res = icmp eq i32 %and, 1
+ ret i1 %res
+}
+
+ at B = external global i32
+declare void @llvm.assume(i1)
+
+; Known bits without a constant
+define i1 @test4(i32 %a) {
+; CHECK-LABEL: @test4
+; CHECK: ret i1 false
+ %b = load i32, i32* @B
+ %b.and = and i32 %b, 1
+ %b.cnd = icmp eq i32 %b.and, 1
+ call void @llvm.assume(i1 %b.cnd)
+
+ %rhs = add i32 %a, %b
+ %and = and i32 %a, %rhs
+ %res = icmp eq i32 %and, 1
+ ret i1 %res
+}
+
+; Negative test - even number
+define i1 @test5(i32 %a) {
+; CHECK-LABEL: @test5
+; CHECK: ret i1 %res
+ %rhs = add i32 %a, 2
+ %and = and i32 %a, %rhs
+ %res = icmp eq i32 %and, 1
+ ret i1 %res
+}
+
+define i1 @test6(i32 %a) {
+; CHECK-LABEL: @test6
+; CHECK: ret i1 false
+ %lhs = add i32 %a, -1
+ %and = and i32 %lhs, %a
+ %res = icmp eq i32 %and, 1
+ ret i1 %res
+}
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