[DAGCombiner][X86] Fold sra (sub AddC, (shl X, N1C)), N1C --> sext (sub AddC1',(trunc X to (width - N1C)))

[craig.topper]

Summary

We already handled this case for add with a constant RHS. A
similar pattern can occur for sub with a constant left hand side.

Test cases use add and a mul representing (neg (shl X, C)) because
that's what I saw in the wild. The mul will be decomposed and then
the new transform can kick in.

Tests have not been committed, but this patch shows the changes.

[craig.topper]

There's still a more general form of this that can show up in address arithmetic that we don't catch https://godbolt.org/z/WKfYvbvxe

[RKSimon]

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

9303

Move N1C earlier? Move the expensive hasOneUse as late as possible

9309

Does this have to be a splat? I think technically only N1C has to be and AddC can be non-uniform except it requires the ShiftC calculation to be refactored for general constant folding. But I'm not certain how often isTruncateFree is going to return true for vector types....

[craig.topper]

Move N1C check.

[craig.topper]

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

9309

I think you're right, but I'd rather not do that in this patch.

[RKSimon]

LGTM

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

9309

OK - please can you add a TODO comment?

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

if (N01C) {
                            N->getValueType(0), Trunc);
       }
     }
   }
 
   // We convert trunc/ext to opposing shifts in IR, but casts may be cheaper.
   //   sra (add (shl X, N1C), AddC), N1C -->
   //   sext (add (trunc X to (width - N1C)), AddC')
-  if (N0.getOpcode() == ISD::ADD && N0.hasOneUse() && N1C &&
-      N0.getOperand(0).getOpcode() == ISD::SHL &&
-      N0.getOperand(0).getOperand(1) == N1 && N0.getOperand(0).hasOneUse()) {
-    if (ConstantSDNode *AddC = isConstOrConstSplat(N0.getOperand(1))) {
-      SDValue Shl = N0.getOperand(0);
-      // Determine what the truncate's type would be and ask the target if that
-      // is a free operation.
+  //   sra (sub AddC, (shl X, N1C)), N1C -->
+  //   sext (sub AddC1',(trunc X to (width - N1C)))
+  if ((N0.getOpcode() == ISD::ADD || N0.getOpcode() == ISD::SUB) && N1C &&
+      N0.hasOneUse()) {
+    bool IsAdd = N0.getOpcode() == ISD::ADD;
+    SDValue Shl = N0.getOperand(IsAdd ? 0 : 1);
+    if (Shl.getOpcode() == ISD::SHL && Shl.getOperand(1) == N1 &&
+        Shl.hasOneUse()) {
+      // TODO: AddC does not need to be a splat.
+      if (ConstantSDNode *AddC =
+              isConstOrConstSplat(N0.getOperand(IsAdd ? 1 : 0))) {
+        // Determine what the truncate's type would be and ask the target if
+        // that is a free operation.
         LLVMContext &Ctx = *DAG.getContext();
         unsigned ShiftAmt = N1C->getZExtValue();
         EVT TruncVT = EVT::getIntegerVT(Ctx, OpSizeInBits - ShiftAmt);
         if (VT.isVector())
           TruncVT = EVT::getVectorVT(Ctx, TruncVT, VT.getVectorElementCount());
 
         // TODO: The simple type check probably belongs in the default hook
         //       implementation and/or target-specific overrides (because
-      //       non-simple types likely require masking when legalized), but that
-      //       restriction may conflict with other transforms.
+        //       non-simple types likely require masking when legalized), but
+        //       that restriction may conflict with other transforms.
         if (TruncVT.isSimple() && isTypeLegal(TruncVT) &&
             TLI.isTruncateFree(VT, TruncVT)) {
           SDLoc DL(N);
           SDValue Trunc = DAG.getZExtOrTrunc(Shl.getOperand(0), DL, TruncVT);
-        SDValue ShiftC = DAG.getConstant(AddC->getAPIntValue().lshr(ShiftAmt).
-                             trunc(TruncVT.getScalarSizeInBits()), DL, TruncVT);
-        SDValue Add = DAG.getNode(ISD::ADD, DL, TruncVT, Trunc, ShiftC);
+          SDValue ShiftC =
+              DAG.getConstant(AddC->getAPIntValue().lshr(ShiftAmt).trunc(
+                                  TruncVT.getScalarSizeInBits()),
+                              DL, TruncVT);
+          SDValue Add;
+          if (IsAdd)
+            Add = DAG.getNode(ISD::ADD, DL, TruncVT, Trunc, ShiftC);
+          else
+            Add = DAG.getNode(ISD::SUB, DL, TruncVT, ShiftC, Trunc);
           return DAG.getSExtOrTrunc(Add, DL, VT);
         }
       }
     }
+  }
 
   // fold (sra x, (trunc (and y, c))) -> (sra x, (and (trunc y), (trunc c))).
   if (N1.getOpcode() == ISD::TRUNCATE &&
       N1.getOperand(0).getOpcode() == ISD::AND) {
     if (SDValue NewOp1 = distributeTruncateThroughAnd(N1.getNode()))
       return DAG.getNode(ISD::SRA, SDLoc(N), VT, N0, NewOp1);
   }
 

llvm/test/CodeGen/X86/shift-combine.ll

 then:
   %t1 = xor i32 %div, -1
   unreachable
 
 if:
   unreachable
 }
 
+; The mul here is the equivalent of (neg (shl X, 32)).
 define i64 @ashr_add_neg_shl_i32(i64 %r) nounwind {
 ; X32-LABEL: ashr_add_neg_shl_i32:
 ; X32:       # %bb.0:
 ; X32-NEXT:    movl $1, %eax
 ; X32-NEXT:    subl {{[0-9]+}}(%esp), %eax
 ; X32-NEXT:    movl %eax, %edx
 ; X32-NEXT:    sarl $31, %edx
 ; X32-NEXT:    retl
 ;
 ; X64-LABEL: ashr_add_neg_shl_i32:
 ; X64:       # %bb.0:
-; X64-NEXT:    shlq $32, %rdi
-; X64-NEXT:    movabsq $4294967296, %rax # imm = 0x100000000
-; X64-NEXT:    subq %rdi, %rax
-; X64-NEXT:    sarq $32, %rax
+; X64-NEXT:    movl $1, %eax
+; X64-NEXT:    subl %edi, %eax
+; X64-NEXT:    cltq
 ; X64-NEXT:    retq
   %conv = mul i64 %r, -4294967296
   %sext = add i64 %conv, 4294967296
   %conv1 = ashr i64 %sext, 32
   ret i64 %conv1
 }
 
+; The mul here is the equivalent of (neg (shl X, 56)).
 define i64 @ashr_add_neg_shl_i8(i64 %r) nounwind {
 ; X32-LABEL: ashr_add_neg_shl_i8:
 ; X32:       # %bb.0:
 ; X32-NEXT:    movl {{[0-9]+}}(%esp), %eax
 ; X32-NEXT:    shll $24, %eax
 ; X32-NEXT:    movl $33554432, %edx # imm = 0x2000000
 ; X32-NEXT:    subl %eax, %edx
 ; X32-NEXT:    movl %edx, %eax
 ; X32-NEXT:    sarl $24, %eax
 ; X32-NEXT:    sarl $31, %edx
 ; X32-NEXT:    retl
 ;
 ; X64-LABEL: ashr_add_neg_shl_i8:
 ; X64:       # %bb.0:
-; X64-NEXT:    shlq $56, %rdi
-; X64-NEXT:    movabsq $144115188075855872, %rax # imm = 0x200000000000000
-; X64-NEXT:    subq %rdi, %rax
-; X64-NEXT:    sarq $56, %rax
+; X64-NEXT:    movb $2, %al
+; X64-NEXT:    subb %dil, %al
+; X64-NEXT:    movsbq %al, %rax
 ; X64-NEXT:    retq
   %conv = mul i64 %r, -72057594037927936
   %sext = add i64 %conv, 144115188075855872
   %conv1 = ashr i64 %sext, 56
   ret i64 %conv1
 }
 
+; The mul here is the equivalent of (neg (shl X, 24)).
 define <4 x i32> @ashr_add_neg_shl_v4i8(<4 x i32> %r) nounwind {
 ; X32-LABEL: ashr_add_neg_shl_v4i8:
 ; X32:       # %bb.0:
-; X32-NEXT:    pushl %ebp
-; X32-NEXT:    pushl %ebx
 ; X32-NEXT:    pushl %edi
 ; X32-NEXT:    pushl %esi
 ; X32-NEXT:    movl {{[0-9]+}}(%esp), %eax
-; X32-NEXT:    movl {{[0-9]+}}(%esp), %ebp
-; X32-NEXT:    movl {{[0-9]+}}(%esp), %ebx
-; X32-NEXT:    movl {{[0-9]+}}(%esp), %esi
-; X32-NEXT:    movl {{[0-9]+}}(%esp), %edx
-; X32-NEXT:    shll $24, %edx
-; X32-NEXT:    shll $24, %esi
-; X32-NEXT:    shll $24, %ebx
-; X32-NEXT:    shll $24, %ebp
-; X32-NEXT:    movl $16777216, %ecx # imm = 0x1000000
-; X32-NEXT:    movl $16777216, %edi # imm = 0x1000000
-; X32-NEXT:    subl %ebp, %edi
-; X32-NEXT:    movl $16777216, %ebp # imm = 0x1000000
-; X32-NEXT:    subl %ebx, %ebp
-; X32-NEXT:    movl $16777216, %ebx # imm = 0x1000000
-; X32-NEXT:    subl %esi, %ebx
-; X32-NEXT:    subl %edx, %ecx
-; X32-NEXT:    sarl $24, %ecx
-; X32-NEXT:    sarl $24, %ebx
-; X32-NEXT:    sarl $24, %ebp
-; X32-NEXT:    sarl $24, %edi
-; X32-NEXT:    movl %edi, 12(%eax)
-; X32-NEXT:    movl %ebp, 8(%eax)
-; X32-NEXT:    movl %ebx, 4(%eax)
-; X32-NEXT:    movl %ecx, (%eax)
+; X32-NEXT:    movb $1, %cl
+; X32-NEXT:    movb $1, %dl
+; X32-NEXT:    subb {{[0-9]+}}(%esp), %dl
+; X32-NEXT:    movsbl %dl, %edx
+; X32-NEXT:    movb $1, %ch
+; X32-NEXT:    subb {{[0-9]+}}(%esp), %ch
+; X32-NEXT:    movsbl %ch, %esi
+; X32-NEXT:    movb $1, %ch
+; X32-NEXT:    subb {{[0-9]+}}(%esp), %ch
+; X32-NEXT:    movsbl %ch, %edi
+; X32-NEXT:    subb {{[0-9]+}}(%esp), %cl
+; X32-NEXT:    movsbl %cl, %ecx
+; X32-NEXT:    movl %ecx, 12(%eax)
+; X32-NEXT:    movl %edi, 8(%eax)
+; X32-NEXT:    movl %esi, 4(%eax)
+; X32-NEXT:    movl %edx, (%eax)
 ; X32-NEXT:    popl %esi
 ; X32-NEXT:    popl %edi
-; X32-NEXT:    popl %ebx
-; X32-NEXT:    popl %ebp
 ; X32-NEXT:    retl $4
 ;
 ; X64-LABEL: ashr_add_neg_shl_v4i8:
 ; X64:       # %bb.0:
 ; X64-NEXT:    pslld $24, %xmm0
 ; X64-NEXT:    movdqa {{.*#+}} xmm1 = [16777216,16777216,16777216,16777216]
 ; X64-NEXT:    psubd %xmm0, %xmm1
 ; X64-NEXT:    psrad $24, %xmm1
 ; X64-NEXT:    movdqa %xmm1, %xmm0
 ; X64-NEXT:    retq
   %conv = mul <4 x i32> %r, <i32 -16777216, i32 -16777216, i32 -16777216, i32 -16777216>
   %sext = add <4 x i32> %conv, <i32 16777216, i32 16777216, i32 16777216, i32 16777216>
   %conv1 = ashr <4 x i32> %sext, <i32 24, i32 24, i32 24, i32 24>
   ret <4 x i32> %conv1
 }