[PATCH] D13253: Tighten known bits for ctpop based on zero input bits
Philip Reames via llvm-commits
llvm-commits at lists.llvm.org
Wed Oct 14 15:44:18 PDT 2015
This revision was automatically updated to reflect the committed changes.
Closed by commit rL250338: Tighten known bits for ctpop based on zero input bits (authored by reames).
Changed prior to commit:
http://reviews.llvm.org/D13253?vs=36654&id=37408#toc
Repository:
rL LLVM
http://reviews.llvm.org/D13253
Files:
llvm/trunk/lib/Analysis/ValueTracking.cpp
llvm/trunk/test/Transforms/InstCombine/ctpop.ll
Index: llvm/trunk/test/Transforms/InstCombine/ctpop.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/ctpop.ll
+++ llvm/trunk/test/Transforms/InstCombine/ctpop.ll
@@ -0,0 +1,45 @@
+; RUN: opt < %s -S -instcombine | FileCheck %s
+
+declare i32 @llvm.ctpop.i32(i32)
+declare i8 @llvm.ctpop.i8(i8)
+declare void @llvm.assume(i1)
+
+define i1 @test1(i32 %arg) {
+; CHECK: @test1
+; CHECK: ret i1 false
+ %and = and i32 %arg, 15
+ %cnt = call i32 @llvm.ctpop.i32(i32 %and)
+ %res = icmp eq i32 %cnt, 9
+ ret i1 %res
+}
+
+define i1 @test2(i32 %arg) {
+; CHECK: @test2
+; CHECK: ret i1 false
+ %and = and i32 %arg, 1
+ %cnt = call i32 @llvm.ctpop.i32(i32 %and)
+ %res = icmp eq i32 %cnt, 2
+ ret i1 %res
+}
+
+define i1 @test3(i32 %arg) {
+; CHECK: @test3
+; CHECK: ret i1 false
+ ;; Use an assume to make all the bits known without triggering constant
+ ;; folding. This is trying to hit a corner case where we have to avoid
+ ;; taking the log of 0.
+ %assume = icmp eq i32 %arg, 0
+ call void @llvm.assume(i1 %assume)
+ %cnt = call i32 @llvm.ctpop.i32(i32 %arg)
+ %res = icmp eq i32 %cnt, 2
+ ret i1 %res
+}
+
+; Negative test for when we know nothing
+define i1 @test4(i8 %arg) {
+; CHECK: @test4
+; CHECK: ret i1 %res
+ %cnt = call i8 @llvm.ctpop.i8(i8 %arg)
+ %res = icmp eq i8 %cnt, 2
+ ret i1 %res
+}
Index: llvm/trunk/lib/Analysis/ValueTracking.cpp
===================================================================
--- llvm/trunk/lib/Analysis/ValueTracking.cpp
+++ llvm/trunk/lib/Analysis/ValueTracking.cpp
@@ -1375,8 +1375,18 @@
break;
}
case Intrinsic::ctpop: {
- unsigned LowBits = Log2_32(BitWidth)+1;
- KnownZero |= APInt::getHighBitsSet(BitWidth, BitWidth - LowBits);
+ computeKnownBits(I->getOperand(0), KnownZero2, KnownOne2, DL,
+ Depth + 1, Q);
+ // We can bound the space the count needs. Also, bits known to be zero
+ // can't contribute to the population.
+ unsigned BitsPossiblySet = BitWidth - KnownZero2.countPopulation();
+ unsigned LeadingZeros =
+ APInt(BitWidth, BitsPossiblySet).countLeadingZeros();
+ assert(LeadingZeros >= 0 && LeadingZeros <= BitWidth);
+ KnownZero |= APInt::getHighBitsSet(BitWidth, LeadingZeros);
+ KnownOne &= ~KnownZero;
+ // TODO: we could bound KnownOne using the lower bound on the number
+ // of bits which might be set provided by popcnt KnownOne2.
break;
}
case Intrinsic::fabs: {
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