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[DAGCombiner] fold select-of-constants to shift 

This reverses the scalar canonicalization proposed in D63382.

Pre: isPowerOf2(C1)
%r = select i1 %cond, i32 C1, i32 0
=>
%z = zext i1 %cond to i32
%r = shl i32 %z, log2(C1)

https://rise4fun.com/Alive/Z50

x86 already tries to fold this pattern, but it isn't done
uniformly, so we still see a diff. AArch64 probably should
enable the TLI hook to benefit too, but that's a follow-on.

llvm-svn: 374397
This commit is contained in:
Sanjay Patel 2019-10-10 17:52:02 +00:00
parent 6a2eff1e68
commit 7b904ce724
2 changed files with 16 additions and 11 deletions
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15
llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
View File

@ -8221,10 +8221,11 @@ SDValue DAGCombiner::foldSelectOfConstants(SDNode *N) {
return Cond;
}
// For any constants that differ by 1, we can transform the select into an
// extend and add. Use a target hook because some targets may prefer to
// transform in the other direction.
// Use a target hook because some targets may prefer to transform in the
// other direction.
if (TLI.convertSelectOfConstantsToMath(VT)) {
// For any constants that differ by 1, we can transform the select into an
// extend and add.
const APInt &C1Val = C1->getAPIntValue();
const APInt &C2Val = C2->getAPIntValue();
if (C1Val - 1 == C2Val) {
@ -8239,6 +8240,14 @@ SDValue DAGCombiner::foldSelectOfConstants(SDNode *N) {
Cond = DAG.getNode(ISD::SIGN_EXTEND, DL, VT, Cond);
return DAG.getNode(ISD::ADD, DL, VT, Cond, N2);
}
// select Cond, Pow2, 0 --> (zext Cond) << log2(Pow2)
if (C1Val.isPowerOf2() && C2Val.isNullValue()) {
if (VT != MVT::i1)
Cond = DAG.getNode(ISD::ZERO_EXTEND, DL, VT, Cond);
SDValue ShAmtC = DAG.getConstant(C1Val.exactLogBase2(), DL, VT);
return DAG.getNode(ISD::SHL, DL, VT, Cond, ShAmtC);
}
}
return SDValue();

12
llvm/test/CodeGen/X86/selectcc-to-shiftand.ll
View File

@ -202,19 +202,15 @@ define i32 @PR31175(i32 %x, i32 %y) {
define i8 @sel_shift_bool_i8(i1 %t) {
; CHECK-NOBMI-LABEL: sel_shift_bool_i8:
; CHECK-NOBMI: # %bb.0:
; CHECK-NOBMI-NEXT: # kill: def $edi killed $edi def $rdi
; CHECK-NOBMI-NEXT: notb %dil
; CHECK-NOBMI-NEXT: shlb $7, %dil
; CHECK-NOBMI-NEXT: leal -128(%rdi), %eax
; CHECK-NOBMI-NEXT: movl %edi, %eax
; CHECK-NOBMI-NEXT: shlb $7, %al
; CHECK-NOBMI-NEXT: # kill: def $al killed $al killed $eax
; CHECK-NOBMI-NEXT: retq
;
; CHECK-BMI-LABEL: sel_shift_bool_i8:
; CHECK-BMI: # %bb.0:
; CHECK-BMI-NEXT: # kill: def $edi killed $edi def $rdi
; CHECK-BMI-NEXT: notb %dil
; CHECK-BMI-NEXT: shlb $7, %dil
; CHECK-BMI-NEXT: leal -128(%rdi), %eax
; CHECK-BMI-NEXT: movl %edi, %eax
; CHECK-BMI-NEXT: shlb $7, %al
; CHECK-BMI-NEXT: # kill: def $al killed $al killed $eax
; CHECK-BMI-NEXT: retq
%shl = select i1 %t, i8 128, i8 0