From: Hal Finkel <hfinkel@anl.gov>
Date: Sun, 6 Sep 2015 05:42:13 +0000 (+0000)
Subject: [SelectionDAG] Swap commutative binops before constant-based folding
X-Git-Url: http://plrg.eecs.uci.edu/git/?p=oota-llvm.git;a=commitdiff_plain;h=987b4b6f61678a58e344fbed4e5c363ab734815c

[SelectionDAG] Swap commutative binops before constant-based folding

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
---

diff --git a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 7ccaaf72c37..3acf749bf3e 100644
--- a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -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);
diff --git a/test/CodeGen/ARM/2012-11-14-subs_carry.ll b/test/CodeGen/ARM/2012-11-14-subs_carry.ll
index 33083303a3d..f7abac06919 100644
--- a/test/CodeGen/ARM/2012-11-14-subs_carry.ll
+++ b/test/CodeGen/ARM/2012-11-14-subs_carry.ll
@@ -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 {