[PATCH] D122572: [X86] combineCarryThroughADD - recognise X86ISD::ADD(AND(X,1),-1) pattern can be folded to X86ISD::BT

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon created this revision.
RKSimon added reviewers: craig.topper, pengfei, spatel, lebedev.ri.
Herald added subscribers: StephenFan, hiraditya.
Herald added a project: All.
RKSimon requested review of this revision.
Herald added a project: LLVM.

As mentioned on D122482 <https://reviews.llvm.org/D122482>, if we've generated a masked overflow test see if we can fold it to X86ISD::BT to feed a X86ISD::ADC/SBB


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D122572

Files:
  llvm/lib/Target/X86/X86ISelLowering.cpp
  llvm/test/CodeGen/X86/addcarry.ll
  llvm/test/CodeGen/X86/combine-adc.ll
  llvm/test/CodeGen/X86/subcarry.ll


Index: llvm/test/CodeGen/X86/subcarry.ll
===================================================================
--- llvm/test/CodeGen/X86/subcarry.ll
+++ llvm/test/CodeGen/X86/subcarry.ll
@@ -349,8 +349,7 @@
 ; CHECK-LABEL: subcarry_fake_carry:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    movq %rdi, %rax
-; CHECK-NEXT:    andb $1, %dl
-; CHECK-NEXT:    addb $-1, %dl
+; CHECK-NEXT:    btl $0, %edx
 ; CHECK-NEXT:    sbbq %rsi, %rax
 ; CHECK-NEXT:    setb %dl
 ; CHECK-NEXT:    retq
Index: llvm/test/CodeGen/X86/combine-adc.ll
===================================================================
--- llvm/test/CodeGen/X86/combine-adc.ll
+++ llvm/test/CodeGen/X86/combine-adc.ll
@@ -68,23 +68,17 @@
 }
 
 ; FIXME: Fail to add (non-overflowing) constants together
-; FIXME: Fail to convert add+lshr+and to BT
 define i32 @adc_merge_constants(i32 %a0) nounwind {
 ; X86-LABEL: adc_merge_constants:
 ; X86:       # %bb.0:
-; X86-NEXT:    movl {{[0-9]+}}(%esp), %eax
-; X86-NEXT:    shrl $11, %eax
-; X86-NEXT:    andb $1, %al
-; X86-NEXT:    addb $-1, %al
+; X86-NEXT:    btl $11, {{[0-9]+}}(%esp)
 ; X86-NEXT:    movl $55, %eax
 ; X86-NEXT:    adcl $-1, %eax
 ; X86-NEXT:    retl
 ;
 ; X64-LABEL: adc_merge_constants:
 ; X64:       # %bb.0:
-; X64-NEXT:    shrl $11, %edi
-; X64-NEXT:    andb $1, %dil
-; X64-NEXT:    addb $-1, %dil
+; X64-NEXT:    btl $11, %edi
 ; X64-NEXT:    movl $55, %eax
 ; X64-NEXT:    adcl $-1, %eax
 ; X64-NEXT:    retq
Index: llvm/test/CodeGen/X86/addcarry.ll
===================================================================
--- llvm/test/CodeGen/X86/addcarry.ll
+++ llvm/test/CodeGen/X86/addcarry.ll
@@ -640,8 +640,7 @@
 ; CHECK-LABEL: addcarry_fake_carry:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    movq %rdi, %rax
-; CHECK-NEXT:    andb $1, %dl
-; CHECK-NEXT:    addb $-1, %dl
+; CHECK-NEXT:    btl $0, %edx
 ; CHECK-NEXT:    adcq %rsi, %rax
 ; CHECK-NEXT:    setb %dl
 ; CHECK-NEXT:    retq
Index: llvm/lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- llvm/lib/Target/X86/X86ISelLowering.cpp
+++ llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -44710,14 +44710,17 @@
 static SDValue combineCarryThroughADD(SDValue EFLAGS, SelectionDAG &DAG) {
   if (EFLAGS.getOpcode() == X86ISD::ADD) {
     if (isAllOnesConstant(EFLAGS.getOperand(1))) {
+      bool FoundAndLSB = false;
       SDValue Carry = EFLAGS.getOperand(0);
       while (Carry.getOpcode() == ISD::TRUNCATE ||
              Carry.getOpcode() == ISD::ZERO_EXTEND ||
              Carry.getOpcode() == ISD::SIGN_EXTEND ||
              Carry.getOpcode() == ISD::ANY_EXTEND ||
              (Carry.getOpcode() == ISD::AND &&
-              isOneConstant(Carry.getOperand(1))))
+              isOneConstant(Carry.getOperand(1)))) {
+        FoundAndLSB |= Carry.getOpcode() == ISD::AND;
         Carry = Carry.getOperand(0);
+      }
       if (Carry.getOpcode() == X86ISD::SETCC ||
           Carry.getOpcode() == X86ISD::SETCC_CARRY) {
         // TODO: Merge this code with equivalent in combineAddOrSubToADCOrSBB?
@@ -44744,10 +44747,16 @@
         }
         // If this is a check of the z flag of an add with 1, switch to the
         // C flag.
-        if (CarryCC == X86::COND_E &&
-            CarryOp1.getOpcode() == X86ISD::ADD &&
+        if (CarryCC == X86::COND_E && CarryOp1.getOpcode() == X86ISD::ADD &&
             isOneConstant(CarryOp1.getOperand(1)))
           return CarryOp1;
+      } else if (FoundAndLSB) {
+        SDValue BitNo = DAG.getConstant(0, SDLoc(Carry), Carry.getValueType());
+        if (Carry.getOpcode() == ISD::SRL) {
+          BitNo = Carry.getOperand(1);
+          Carry = Carry.getOperand(0);
+        }
+        return DAG.getNode(X86ISD::BT, SDLoc(Carry), MVT::i32, Carry, BitNo);
       }
     }
   }


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