[llvm] r354514 - [SelectionDAG] Teach GetDemandedBits to look at the known zeros of the LHS when handling ISD::AND
Craig Topper via llvm-commits
llvm-commits at lists.llvm.org
Wed Feb 20 12:52:26 PST 2019
Author: ctopper
Date: Wed Feb 20 12:52:26 2019
New Revision: 354514
URL: http://llvm.org/viewvc/llvm-project?rev=354514&view=rev
Log:
[SelectionDAG] Teach GetDemandedBits to look at the known zeros of the LHS when handling ISD::AND
If the LHS has known zeros, then the RHS immediate mask might have been simplified to remove those bits.
This patch adds a call to computeKnownBits to get the known zeroes to handle that possibility. I left an early out to skip the call if all of the demanded bits are set in the mask.
Differential Revision: https://reviews.llvm.org/D58464
Modified:
llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
llvm/trunk/test/CodeGen/X86/bt.ll
Modified: llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp?rev=354514&r1=354513&r2=354514&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp (original)
+++ llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp Wed Feb 20 12:52:26 2019
@@ -2102,9 +2102,13 @@ SDValue SelectionDAG::GetDemandedBits(SD
break;
case ISD::AND: {
// X & -1 -> X (ignoring bits which aren't demanded).
- ConstantSDNode *AndVal = isConstOrConstSplat(V.getOperand(1));
- if (AndVal && Mask.isSubsetOf(AndVal->getAPIntValue()))
- return V.getOperand(0);
+ // Also handle the case where masked out bits in X are known to be zero.
+ if (ConstantSDNode *RHSC = isConstOrConstSplat(V.getOperand(1))) {
+ const APInt &AndVal = RHSC->getAPIntValue();
+ if (Mask.isSubsetOf(AndVal) ||
+ Mask.isSubsetOf(computeKnownBits(V.getOperand(0)).Zero | AndVal))
+ return V.getOperand(0);
+ }
break;
}
case ISD::ANY_EXTEND: {
Modified: llvm/trunk/test/CodeGen/X86/bt.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/bt.ll?rev=354514&r1=354513&r2=354514&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/X86/bt.ll (original)
+++ llvm/trunk/test/CodeGen/X86/bt.ll Wed Feb 20 12:52:26 2019
@@ -1153,7 +1153,6 @@ define zeroext i1 @demanded_with_known_z
; X86-NEXT: movl {{[0-9]+}}(%esp), %eax
; X86-NEXT: movb {{[0-9]+}}(%esp), %cl
; X86-NEXT: shlb $2, %cl
-; X86-NEXT: andb $28, %cl
; X86-NEXT: movzbl %cl, %ecx
; X86-NEXT: btl %ecx, %eax
; X86-NEXT: setb %al
@@ -1162,7 +1161,6 @@ define zeroext i1 @demanded_with_known_z
; X64-LABEL: demanded_with_known_zeroes:
; X64: # %bb.0: # %entry
; X64-NEXT: shlb $2, %dil
-; X64-NEXT: andb $28, %dil
; X64-NEXT: movzbl %dil, %eax
; X64-NEXT: btl %eax, %esi
; X64-NEXT: setb %al
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