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[SelectionDAG] Swap commutative binops before constant-based folding

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In searching for a fix for the underlying code-quality bug highlighted by
r246937 (that SDAG simplification can lead to us generating an ISD::OR node
with a constant zero LHS), I ran across this:

We generically canonicalize commutative binary-operation nodes in SDAG getNode
so that, if only one operand is a constant, it will be on the RHS.  However, we
were doing this only after a bunch of constant-based simplification checks that
all assume this canonical form (that any constant will be on the RHS). Moving
the operand-swapping canonicalization prior to these checks seems like the
right thing to do (and, as it turns out, causes SDAG to completely fold away the
computation in test/CodeGen/ARM/2012-11-14-subs_carry.ll, just like InstCombine
would do).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246938 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Hal Finkel 2015-09-06 05:42:13 +00:00
parent 9f11e5da34
commit 987b4b6f61
2 changed files with 14 additions and 9 deletions
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13
lib/CodeGen/SelectionDAG/SelectionDAG.cpp
View File

@ -3348,6 +3348,13 @@ SDValue SelectionDAG::getNode(unsigned Opcode, SDLoc DL, EVT VT, SDValue N1,
SDValue N2, const SDNodeFlags *Flags) {
ConstantSDNode *N1C = dyn_cast<ConstantSDNode>(N1);
ConstantSDNode *N2C = dyn_cast<ConstantSDNode>(N2);
// Canonicalize constant to RHS if commutative.
if (N1C && !N2C && isCommutativeBinOp(Opcode)) {
std::swap(N1C, N2C);
std::swap(N1, N2);
}
switch (Opcode) {
default: break;
case ISD::TokenFactor:
@ -3692,12 +3699,6 @@ SDValue SelectionDAG::getNode(unsigned Opcode, SDLoc DL, EVT VT, SDValue N1,
FoldConstantArithmetic(Opcode, DL, VT, N1.getNode(), N2.getNode()))
return SV;
// Canonicalize constant to RHS if commutative.
if (N1C && !N2C && isCommutativeBinOp(Opcode)) {
std::swap(N1C, N2C);
std::swap(N1, N2);
}
// Constant fold FP operations.
bool HasFPExceptions = TLI->hasFloatingPointExceptions();
ConstantFPSDNode *N1CFP = dyn_cast<ConstantFPSDNode>(N1);

10
test/CodeGen/ARM/2012-11-14-subs_carry.ll
View File

@ -1,10 +1,14 @@
; RUN: llc < %s -mtriple=thumbv7-apple-ios -arm-atomic-cfg-tidy=0 | FileCheck %s
;CHECK-LABEL: foo:
;CHECK: adds
;CHECK-NEXT: adc
;CHECK-NEXT: bx
;CHECK: movs r0, #0
;CHECK-NEXT: bx lr
; Note: This test case originally checked, per r167963, for:
; adds
; adc
; bx
; But SDAG now, like InstCombine, can fold everything away.
;rdar://12028498
define i32 @foo() nounwind ssp {