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[SelectionDAG] Improve known bits support for CTPOP. 

This is based on the same concept from ValueTracking's version of computeKnownBits.

llvm-svn: 302110
This commit is contained in:
Craig Topper 2017-05-04 04:33:27 +00:00
parent bdfe90050b
commit d4d09fd73d
2 changed files with 18 additions and 1 deletions
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5
llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
View File

@ -2376,7 +2376,10 @@ void SelectionDAG::computeKnownBits(SDValue Op, KnownBits &Known,
break;
}
case ISD::CTPOP: {
Known.Zero.setBitsFrom(Log2_32(BitWidth)+1);
computeKnownBits(Op.getOperand(0), Known2, DemandedElts, Depth + 1);
// If we know some of the bits are zero, they can't be one.
unsigned PossibleOnes = BitWidth - Known2.Zero.countPopulation();
Known.Zero.setBitsFrom(Log2_32(PossibleOnes) + 1);
break;
}
case ISD::LOAD: {

14
llvm/test/CodeGen/X86/ctpop-combine.ll
View File

@ -1,6 +1,7 @@
; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
; RUN: llc < %s -mtriple=x86_64-unknown -mcpu=corei7 | FileCheck %s
declare i8 @llvm.ctpop.i8(i8) nounwind readnone
declare i64 @llvm.ctpop.i64(i64) nounwind readnone
define i32 @test1(i64 %x) nounwind readnone {
@ -48,3 +49,16 @@ define i32 @test3(i64 %x) nounwind readnone {
%conv = zext i1 %cmp to i32
ret i32 %conv
}
define i8 @test4(i8 %x) nounwind readnone {
; CHECK-LABEL: test4:
; CHECK: # BB#0:
; CHECK-NEXT: andl $127, %edi
; CHECK-NEXT: popcntw %di, %ax
; CHECK-NEXT: # kill: %AL<def> %AL<kill> %AX<kill>
; CHECK-NEXT: retq
%x2 = and i8 %x, 127
%count = tail call i8 @llvm.ctpop.i8(i8 %x2)
%and = and i8 %count, 7
ret i8 %and
}