[PATCH] D57281: [X86] combineCarryThroughADD - add support for X86::COND_A commutations (PR24545)

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon created this revision.
RKSimon added reviewers: craig.topper, spatel, andreadb.

As discussed on PR24545, we should try to commute X86::COND_A 'icmp ugt' cases to X86::COND_B 'icmp ult' to more optimally bind the carry flag output to a SBB instruction.


Repository:
  rL LLVM

https://reviews.llvm.org/D57281

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/combine-sbb.ll


Index: test/CodeGen/X86/combine-sbb.ll
===================================================================
--- test/CodeGen/X86/combine-sbb.ll
+++ test/CodeGen/X86/combine-sbb.ll
@@ -129,19 +129,15 @@
 ; X86:       # %bb.0:
 ; X86-NEXT:    movl {{[0-9]+}}(%esp), %eax
 ; X86-NEXT:    movl {{[0-9]+}}(%esp), %ecx
-; X86-NEXT:    movl (%ecx), %edx
-; X86-NEXT:    cmpl {{[0-9]+}}(%esp), %edx
-; X86-NEXT:    seta %dl
-; X86-NEXT:    addb $-1, %dl
+; X86-NEXT:    movl {{[0-9]+}}(%esp), %edx
+; X86-NEXT:    cmpl (%ecx), %edx
 ; X86-NEXT:    sbbl 4(%ecx), %eax
 ; X86-NEXT:    setb %al
 ; X86-NEXT:    retl
 ;
 ; X64-LABEL: PR24545:
 ; X64:       # %bb.0:
-; X64-NEXT:    cmpl %edi, (%rdx)
-; X64-NEXT:    seta %al
-; X64-NEXT:    addb $-1, %al
+; X64-NEXT:    cmpl (%rdx), %edi
 ; X64-NEXT:    sbbl 4(%rdx), %esi
 ; X64-NEXT:    setb %al
 ; X64-NEXT:    retq
Index: lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- lib/Target/X86/X86ISelLowering.cpp
+++ lib/Target/X86/X86ISelLowering.cpp
@@ -34862,7 +34862,7 @@
 // When legalizing carry, we create carries via add X, -1
 // If that comes from an actual carry, via setcc, we use the
 // carry directly.
-static SDValue combineCarryThroughADD(SDValue EFLAGS) {
+static SDValue combineCarryThroughADD(SDValue EFLAGS, SelectionDAG &DAG) {
   if (EFLAGS.getOpcode() == X86ISD::ADD) {
     if (isAllOnesConstant(EFLAGS.getOperand(1))) {
       SDValue Carry = EFLAGS.getOperand(0);
@@ -34875,8 +34875,27 @@
         Carry = Carry.getOperand(0);
       if (Carry.getOpcode() == X86ISD::SETCC ||
           Carry.getOpcode() == X86ISD::SETCC_CARRY) {
-        if (Carry.getConstantOperandVal(0) == X86::COND_B)
-          return Carry.getOperand(1);
+        // TODO: Merge this code with equivalent in combineAddOrSubToADCOrSBB?
+        uint64_t CarryCC = Carry.getConstantOperandVal(0);
+        SDValue CarryOp1 = Carry.getOperand(1);
+        if (CarryCC == X86::COND_B)
+          return CarryOp1;
+        if (CarryCC == X86::COND_A) {
+          // Try to convert COND_A into COND_B in an attempt to facilitate
+          // materializing "setb reg".
+          //
+          // Do not flip "e > c", where "c" is a constant, because Cmp
+          // instruction cannot take an immediate as its first operand.
+          //
+          if (CarryOp1.getOpcode() == X86ISD::SUB && CarryOp1.hasOneUse() &&
+              CarryOp1.getValueType().isInteger() &&
+              !isa<ConstantSDNode>(CarryOp1.getOperand(1))) {
+            SDValue SubCommute =
+                DAG.getNode(X86ISD::SUB, SDLoc(CarryOp1), CarryOp1->getVTList(),
+                            CarryOp1.getOperand(1), CarryOp1.getOperand(0));
+            return SDValue(SubCommute.getNode(), CarryOp1.getResNo());
+          }
+        }
       }
     }
   }
@@ -34891,7 +34910,7 @@
                                   SelectionDAG &DAG,
                                   const X86Subtarget &Subtarget) {
   if (CC == X86::COND_B)
-    if (SDValue Flags = combineCarryThroughADD(EFLAGS))
+    if (SDValue Flags = combineCarryThroughADD(EFLAGS, DAG))
       return Flags;
 
   if (SDValue R = checkBoolTestSetCCCombine(EFLAGS, CC))
@@ -40544,7 +40563,7 @@
 }
 
 static SDValue combineSBB(SDNode *N, SelectionDAG &DAG) {
-  if (SDValue Flags = combineCarryThroughADD(N->getOperand(2))) {
+  if (SDValue Flags = combineCarryThroughADD(N->getOperand(2), DAG)) {
     MVT VT = N->getSimpleValueType(0);
     SDVTList VTs = DAG.getVTList(VT, MVT::i32);
     return DAG.getNode(X86ISD::SBB, SDLoc(N), VTs,
@@ -40578,7 +40597,7 @@
     return DCI.CombineTo(N, Res1, CarryOut);
   }
 
-  if (SDValue Flags = combineCarryThroughADD(N->getOperand(2))) {
+  if (SDValue Flags = combineCarryThroughADD(N->getOperand(2), DAG)) {
     MVT VT = N->getSimpleValueType(0);
     SDVTList VTs = DAG.getVTList(VT, MVT::i32);
     return DAG.getNode(X86ISD::ADC, SDLoc(N), VTs,


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