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[AArch64] Avoid materializing constant 1 when generating cneg instructions.

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Instead of

 cmp w0, #1
 orr w8, wzr, #0x1
 cneg w0, w8, ne

we now generate

 cmp w0, #1
 csinv w0, w0, wzr, eq

PR28965

llvm-svn: 285217
This commit is contained in:
Chad Rosier 2016-10-26 18:15:32 +00:00
parent bc67d8a523
commit c0750fd641
2 changed files with 24 additions and 2 deletions
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15
lib/Target/AArch64/AArch64ISelLowering.cpp
View File

@ -4054,8 +4054,9 @@ SDValue AArch64TargetLowering::LowerSELECT_CC(ISD::CondCode CC, SDValue LHS,
// Avoid materializing a constant when possible by reusing a known value in
// a register. However, don't perform this optimization if the known value
// is one, zero or negative one. We can always materialize these values
// using CSINC, CSEL and CSINV with wzr/xzr as the FVal, respectively.
// is one, zero or negative one in the case of a CSEL. We can always
// materialize these values using CSINC, CSEL and CSINV with wzr/xzr as the
// FVal, respectively.
ConstantSDNode *RHSVal = dyn_cast<ConstantSDNode>(RHS);
if (Opcode == AArch64ISD::CSEL && RHSVal && !RHSVal->isOne() &&
!RHSVal->isNullValue() && !RHSVal->isAllOnesValue()) {
@ -4066,6 +4067,16 @@ SDValue AArch64TargetLowering::LowerSELECT_CC(ISD::CondCode CC, SDValue LHS,
TVal = LHS;
else if (CFVal && CFVal == RHSVal && AArch64CC == AArch64CC::NE)
FVal = LHS;
} else if (Opcode == AArch64ISD::CSNEG && RHSVal && RHSVal->isOne()) {
assert (CTVal && CFVal && "Expected constant operands for CSNEG.");
// Use a CSINV to transform "a == C ? 1 : -1" to "a == C ? a : -1" to
// avoid materializing C.
AArch64CC::CondCode AArch64CC = changeIntCCToAArch64CC(CC);
if (CTVal == RHSVal && AArch64CC == AArch64CC::EQ) {
Opcode = AArch64ISD::CSINV;
TVal = LHS;
FVal = DAG.getConstant(0, dl, FVal.getValueType());
}
}
SDValue CCVal;

11
test/CodeGen/AArch64/cond-sel-value-prop.ll
View File

@ -97,3 +97,14 @@ define i64 @test9(i64 %x) {
%res = select i1 %cmp, i64 7, i64 -1
ret i64 %res
}
; Rather than use a CNEG, use a CSINV to transform "a == 1 ? 1 : -1" to
; "a == 1 ? a : -1" to avoid materializing a constant.
; CHECK-LABEL: test10:
; CHECK: cmp w[[REG:[0-9]]], #1
; CHECK: cneg w0, w[[REG]], ne
define i32 @test10(i32 %x) {
%cmp = icmp eq i32 %x, 1
%res = select i1 %cmp, i32 1, i32 -1
ret i32 %res
}