[DAGCombine][ARM] (sub Carry, X) -> (addcarry (sub 0, X), 0, Carry) fold

[lebedev.ri]

Summary

DAGCombiner::visitADDLikeCommutative() already has a sibling fold: (add X, Carry) -> (addcarry X, 0, Carry)
This fold, as suggested by @efriedma, helps recover from some of the regressions of D62266

[efriedma]

It looks like we don't have great test coverage here. In general, we probably need to restrict getAsCarry depending on whether the intended use is an an addition or subtraction, at least for ARM.

test/CodeGen/ARM/addsubcarry-promotion.ll

28

The redundant sxth here is unfortunate, but probably orthogonal. Maybe a missing SimplifyDemandedBits case?

[lebedev.ri]

In D62392#1516829, @efriedma wrote:

It looks like we don't have great test coverage here.

Yes, indeed, that is the only affected test in the entire check-llvm-codegen.

In D62392#1516829, @efriedma wrote:

In general, we probably need to restrict getAsCarry depending on whether the intended use is an an addition or subtraction, at least for ARM.

I'm sorry, but i'm again not sure what that means (:
Restrict how?

[efriedma]

I mean, we basically need two methods: getAsAddCarry (carry produced by UADDO/ADDCARRY) and getAsSubCarry (carry produced by USUBO/SUBCARRY), depending on how you're planning to use the carry bit, so we don't end up with same sort of mismatch we ran into before on ARM.

[lebedev.ri]

In D62392#1516927, @efriedma wrote:

I mean, we basically need two methods: getAsAddCarry (carry produced by UADDO/ADDCARRY) and getAsSubCarry (carry produced by USUBO/SUBCARRY), depending on how you're planning to use the carry bit, so we don't end up with same sort of mismatch we ran into before on ARM.

Thank you for the explanation.

Is that required to be part of the follow-up fixes for D62266?
It sounds, while not unrelated, not exactly specific to the regressions there.
I don't know much about these 'carry' ops, so unless it's required to
fix those regressions, it might not be best for me to look into that..

[lebedev.ri]

Rebased, ping @efriedma.

[efriedma]

If you don't want to continue working on this, that's okay, I think. D62450 looks like it's the more interesting fix.

[lebedev.ri]

In D62392#1525650, @efriedma wrote:

If you don't want to continue working on this, that's okay, I think.

I'm just really unfamiliar with carry-nodes (and with ARM),
so it is not obvious to me what needs fixing, and in what way,
if there are no existing tests that regress due to my change.

In D62392#1525650, @efriedma wrote:

D62450 looks like it's the more interesting fix.

These two are intertwined.
D62450 folds constant from setcc into parent addcarry/subcarry.
But without this patch there is no addcarry/subcarry...

[efriedma]

Oh, I see...

I'll see if I can find some time to add some more testcases to the tree.

[lebedev.ri]

In D62392#1525658, @efriedma wrote:

Oh, I see...

I'll see if I can find some time to add some more testcases to the tree.

Yes, please do, that would be helpful.

[deadalnix]

Maybe it is worth adding some platform dependent check to actually make sure turning the carry into a scalar is expensive? Or is it a reasonable assumption to make that it expensive on all plateforms?

In any case, this is more likely than not that this will optimize better down the road anyways, so maybe, if such plateform exist, we may want to delegate the cleanup to plateform specific transforms.

I think it would be beneficial to have an X86 test case for this pattern.

[lebedev.ri]

In D62392#1587798, @deadalnix wrote:

Maybe it is worth adding some platform dependent check to actually make sure turning the carry into a scalar is expensive? Or is it a reasonable assumption to make that it expensive on all plateforms?

I honestly don't know the answers to these questions, carry nodes are not my strong side,
If you want (given your recent carry patches), you can totally take this patch over.

In any case, this is more likely than not that this will optimize better down the road anyways, so maybe, if such plateform exist, we may want to delegate the cleanup to plateform specific transforms.

I think it would be beneficial to have an X86 test case for this pattern.

[deadalnix]

@lebedev.ri I think these are reasonable assumptions. Just add a test case for X86 and this is good to go as far as I'm concerned.

[deadalnix]

Ping ?

[lebedev.ri]

In D62392#1592596, @deadalnix wrote:

@lebedev.ri I think these are reasonable assumptions. Just add a test case for X86 and this is good to go as far as I'm concerned.

Added x86 test and landed.

lib/CodeGen/SelectionDAG/DAGCombiner.cpp

if (!LegalOperations && N1.getOpcode() == ISD::SRL && N1.hasOneUse()) {
     SDValue ShAmt = N1.getOperand(1);
     ConstantSDNode *ShAmtC = isConstOrConstSplat(ShAmt);
     if (ShAmtC && ShAmtC->getZExtValue() == N1.getScalarValueSizeInBits() - 1) {
       SDValue SRA = DAG.getNode(ISD::SRA, DL, VT, N1.getOperand(0), ShAmt);
       return DAG.getNode(ISD::ADD, DL, VT, N0, SRA);
     }
   }
 
+  if (TLI.isOperationLegalOrCustom(ISD::ADDCARRY, VT)) {
+    // (sub Carry, X)  ->  (addcarry (sub 0, X), 0, Carry)
+    if (SDValue Carry = getAsCarry(TLI, N0)) {
+      SDValue X = N1;
+      SDValue Zero = DAG.getConstant(0, DL, VT);
+      SDValue NegX = DAG.getNode(ISD::SUB, DL, VT, Zero, X);
+      return DAG.getNode(ISD::ADDCARRY, DL,
+                         DAG.getVTList(VT, Carry.getValueType()), NegX, Zero,
+                         Carry);
+    }
+  }
+
   return SDValue();
 }
 
 SDValue DAGCombiner::visitSUBSAT(SDNode *N) {
   SDValue N0 = N->getOperand(0);
   SDValue N1 = N->getOperand(1);
   EVT VT = N0.getValueType();
   SDLoc DL(N);

test/CodeGen/ARM/addsubcarry-promotion.ll

 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc -O2 -mtriple armv7a < %s | FileCheck --check-prefixes=ARM,ARMV7A %s
 
 ; RUN: llc -O2 -mtriple thumbv6m < %s | FileCheck --check-prefixes=THUMB1,THUMBV6M %s
 ; RUN: llc -O2 -mtriple thumbv8m.base < %s | FileCheck --check-prefixes=THUMB1,THUMBV8M-BASE %s
 
 ; RUN: llc -O2 -mtriple thumbv7a < %s | FileCheck --check-prefixes=THUMB,THUMBV7A %s
 ; RUN: llc -O2 -mtriple thumbv8m.main < %s | FileCheck --check-prefixes=THUMB,THUMBV8M-MAIN %s
 
 define void @fn1(i32 %a, i32 %b, i32 %c) local_unnamed_addr #0 {
 ; ARM-LABEL: fn1:
 ; ARM:       @ %bb.0: @ %entry
+; ARM-NEXT:    rsb r2, r2, #0
 ; ARM-NEXT:    adds r0, r1, r0
-; ARM-NEXT:    mov r3, #0
-; ARM-NEXT:    adc r0, r3, #0
 ; ARM-NEXT:    movw r1, #65535
-; ARM-NEXT:    sub r0, r0, r2
+; ARM-NEXT:    sxth r2, r2
+; ARM-NEXT:    adc r0, r2, #0
 ; ARM-NEXT:    uxth r0, r0
 ; ARM-NEXT:    cmp r0, r1
 ; ARM-NEXT:    bxeq lr
 ; ARM-NEXT:  .LBB0_1: @ %for.cond
 ; ARM-NEXT:    @ =>This Inner Loop Header: Depth=1
 ; ARM-NEXT:    b .LBB0_1
 ;
 ; THUMBV6M-LABEL: fn1:
 ; THUMBV6M:       @ %bb.0: @ %entry
+; THUMBV6M-NEXT:    rsbs r2, r2, #0
+; THUMBV6M-NEXT:    sxth r2, r2
 ; THUMBV6M-NEXT:    movs r3, #0
 ; THUMBV6M-NEXT:    adds r0, r1, r0
-; THUMBV6M-NEXT:    adcs r3, r3
-; THUMBV6M-NEXT:    subs r0, r3, r2
-; THUMBV6M-NEXT:    uxth r0, r0
+; THUMBV6M-NEXT:    adcs r3, r2
+; THUMBV6M-NEXT:    uxth r0, r3
 ; THUMBV6M-NEXT:    ldr r1, .LCPI0_0
 ; THUMBV6M-NEXT:    cmp r0, r1
 ; THUMBV6M-NEXT:    beq .LBB0_2
 ; THUMBV6M-NEXT:  .LBB0_1: @ %for.cond
 ; THUMBV6M-NEXT:    @ =>This Inner Loop Header: Depth=1
 ; THUMBV6M-NEXT:    b .LBB0_1
 ; THUMBV6M-NEXT:  .LBB0_2: @ %if.end
 ; THUMBV6M-NEXT:    bx lr
 ; THUMBV6M-NEXT:    .p2align 2
 ; THUMBV6M-NEXT:  @ %bb.3:
 ; THUMBV6M-NEXT:  .LCPI0_0:
 ; THUMBV6M-NEXT:    .long 65535 @ 0xffff
 ;
 ; THUMBV8M-BASE-LABEL: fn1:
 ; THUMBV8M-BASE:       @ %bb.0: @ %entry
+; THUMBV8M-BASE-NEXT:    rsbs r2, r2, #0
+; THUMBV8M-BASE-NEXT:    sxth r2, r2
 ; THUMBV8M-BASE-NEXT:    movs r3, #0
 ; THUMBV8M-BASE-NEXT:    adds r0, r1, r0
-; THUMBV8M-BASE-NEXT:    adcs r3, r3
-; THUMBV8M-BASE-NEXT:    subs r0, r3, r2
-; THUMBV8M-BASE-NEXT:    uxth r0, r0
+; THUMBV8M-BASE-NEXT:    adcs r3, r2
+; THUMBV8M-BASE-NEXT:    uxth r0, r3
 ; THUMBV8M-BASE-NEXT:    movw r1, #65535
 ; THUMBV8M-BASE-NEXT:    cmp r0, r1
 ; THUMBV8M-BASE-NEXT:    beq .LBB0_2
 ; THUMBV8M-BASE-NEXT:  .LBB0_1: @ %for.cond
 ; THUMBV8M-BASE-NEXT:    @ =>This Inner Loop Header: Depth=1
 ; THUMBV8M-BASE-NEXT:    b .LBB0_1
 ; THUMBV8M-BASE-NEXT:  .LBB0_2: @ %if.end
 ; THUMBV8M-BASE-NEXT:    bx lr
 ;
 ; THUMB-LABEL: fn1:
 ; THUMB:       @ %bb.0: @ %entry
+; THUMB-NEXT:    rsbs r2, r2, #0
 ; THUMB-NEXT:    adds r0, r0, r1
-; THUMB-NEXT:    mov.w r3, #0
-; THUMB-NEXT:    adc r0, r3, #0
 ; THUMB-NEXT:    movw r1, #65535
-; THUMB-NEXT:    subs r0, r0, r2
+; THUMB-NEXT:    sxth r2, r2
+; THUMB-NEXT:    adc r0, r2, #0
 ; THUMB-NEXT:    uxth r0, r0
 ; THUMB-NEXT:    cmp r0, r1
 ; THUMB-NEXT:    it eq
 ; THUMB-NEXT:    bxeq lr
 ; THUMB-NEXT:  .LBB0_1: @ %for.cond
 ; THUMB-NEXT:    @ =>This Inner Loop Header: Depth=1
 ; THUMB-NEXT:    b .LBB0_1
 entry: