[DAGCombine] fold (sext (sext_inreg x)) -> (sext (trunc x))

[deadalnix]

Summary

This fixes a regression introduced by D127115 in test/CodeGen/PowerPC/store-forward-be64.ll

[RKSimon]

I'm a bit worried this will end up fighting the existing "(sext (truncate x)) -> (sextinreg x)" fold

[deadalnix]

Yes, it shouldn't in itself, but throw another transform in the loop, and it may well be.

Is there another approach you are thinking of? I thought about doing just (sext (sext_inreg (trunc x))) -> (sext (trunc x)) but that seemed too topical.

[RKSimon]

If you're after test cases: https://gcc.godbolt.org/z/dPzYvz6dW - you might want to look to see why this isn't helping

This is from x86/sdiv_fix.ll which is currently repeating sign extensions: https://github.com/llvm/llvm-project/blob/main/llvm/test/CodeGen/X86/sdiv_fix.ll#L68:L71

      t2: i32,ch = CopyFromReg t0, Register:i32 %0
    t21: i16 = truncate t2
  t22: i16 = sign_extend_inreg t21, ValueType:ch:i8
t53: i32 = sign_extend t22

; X64-NEXT:    movsbl %dil, %eax
; X64-NEXT:    movsbl %sil, %ecx
; X64-NEXT:    movswl %cx, %esi  ; <-- ECX IS ALREADY SIGN-EXTENDED FROM i8
; X64-NEXT:    movswl %ax, %ecx  ; <-- EAX IS ALREADY SIGN-EXTENDED FROM i8

[deadalnix]

Specialize the transform, which avoids problems with the transform creating an infinite loop.

Make sure we materialize improvements in sdiv_fix.

[RKSimon]

LGTM - cheers

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

SDValue DAGCombiner::visitSIGN_EXTEND(SDNode *N) {
   if (SDValue Res = tryToFoldExtendOfConstant(N, TLI, DAG, LegalTypes))
     return Res;
 
   // fold (sext (sext x)) -> (sext x)
   // fold (sext (aext x)) -> (sext x)
   if (N0.getOpcode() == ISD::SIGN_EXTEND || N0.getOpcode() == ISD::ANY_EXTEND)
     return DAG.getNode(ISD::SIGN_EXTEND, DL, VT, N0.getOperand(0));
 
+  // fold (sext (sext_inreg x)) -> (sext (trunc x))
+  if (N0.getOpcode() == ISD::SIGN_EXTEND_INREG) {
+    SDValue N00 = N0.getOperand(0);
+    EVT ExtVT = cast<VTSDNode>(N0->getOperand(1))->getVT();
+    if (N00.getOpcode() == ISD::TRUNCATE && (!LegalOperations || TLI.isTypeLegal(ExtVT))) {
+      SDValue T = DAG.getNode(ISD::TRUNCATE, DL, ExtVT, N00.getOperand(0));
+      return DAG.getNode(ISD::SIGN_EXTEND, DL, VT, T);
+    }
+  }
+
   if (N0.getOpcode() == ISD::TRUNCATE) {
     // fold (sext (truncate (load x))) -> (sext (smaller load x))
     // fold (sext (truncate (srl (load x), c))) -> (sext (smaller load (x+c/n)))
     if (SDValue NarrowLoad = reduceLoadWidth(N0.getNode())) {
       SDNode *oye = N0.getOperand(0).getNode();
       if (NarrowLoad.getNode() != N0.getNode()) {
         CombineTo(N0.getNode(), NarrowLoad);
         // CombineTo deleted the truncate, if needed, but not what's under it.

llvm/test/CodeGen/X86/sdiv_fix.ll

 ; X86-NEXT:    retl
   %tmp = call i16 @llvm.sdiv.fix.i16(i16 %x, i16 %y, i32 7)
   ret i16 %tmp
 }
 
 define i16 @func2(i8 %x, i8 %y) nounwind {
 ; X64-LABEL: func2:
 ; X64:       # %bb.0:
-; X64-NEXT:    movsbl %dil, %eax
-; X64-NEXT:    movsbl %sil, %ecx
-; X64-NEXT:    movswl %cx, %esi
-; X64-NEXT:    movswl %ax, %ecx
+; X64-NEXT:    movsbl %sil, %esi
+; X64-NEXT:    movsbl %dil, %ecx
 ; X64-NEXT:    shll $14, %ecx
 ; X64-NEXT:    movl %ecx, %eax
 ; X64-NEXT:    cltd
 ; X64-NEXT:    idivl %esi
 ; X64-NEXT:    # kill: def $eax killed $eax def $rax
 ; X64-NEXT:    leal -1(%rax), %edi
 ; X64-NEXT:    testl %esi, %esi
 ; X64-NEXT:    sets %sil

llvm/test/CodeGen/X86/sdiv_fix_sat.ll

; X86-NEXT:    retl
   %tmp = call i16 @llvm.sdiv.fix.sat.i16(i16 %x, i16 %y, i32 7)
   ret i16 %tmp
 }
 
 define i16 @func2(i8 %x, i8 %y) nounwind {
 ;
 ; X64-LABEL: func2:
 ; X64:       # %bb.0:
-; X64-NEXT:    movsbl %dil, %eax
-; X64-NEXT:    movsbl %sil, %ecx
-; X64-NEXT:    movswl %cx, %esi
-; X64-NEXT:    movswl %ax, %ecx
+; X64-NEXT:    movsbl %sil, %esi
+; X64-NEXT:    movsbl %dil, %ecx
 ; X64-NEXT:    shll $14, %ecx
 ; X64-NEXT:    movl %ecx, %eax
 ; X64-NEXT:    cltd
 ; X64-NEXT:    idivl %esi
 ; X64-NEXT:    # kill: def $eax killed $eax def $rax
 ; X64-NEXT:    leal -1(%rax), %edi
 ; X64-NEXT:    testl %esi, %esi
 ; X64-NEXT:    sets %sil