[PATCH] D62392: [DAGCombine][ARM] (sub Carry, X) -> (addcarry (sub 0, X), 0, Carry) fold

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri created this revision.
lebedev.ri added a reviewer: efriedma.
lebedev.ri added a project: LLVM.
Herald added subscribers: kristof.beyls, javed.absar.
lebedev.ri added a parent revision: D62266: [DAGCombine][X86][AArch64][ARM] (C - x) + y  ->  (y - x) + C  fold.
lebedev.ri added a child revision: D62294: [DAGCombine] (x - C) - y  ->  (x - y) - C  fold.

`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 <https://reviews.llvm.org/D62266>


Repository:
  rL LLVM

https://reviews.llvm.org/D62392

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/ARM/addsubcarry-promotion.ll


Index: test/CodeGen/ARM/addsubcarry-promotion.ll
===================================================================
--- test/CodeGen/ARM/addsubcarry-promotion.ll
+++ test/CodeGen/ARM/addsubcarry-promotion.ll
@@ -10,11 +10,11 @@
 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
@@ -24,11 +24,12 @@
 ;
 ; 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
@@ -44,11 +45,12 @@
 ;
 ; 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
@@ -60,11 +62,11 @@
 ;
 ; 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
Index: lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -3024,6 +3024,18 @@
     }
   }
 
+  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();
 }
 


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