[llvm] [Codegen][LegalizeIntegerTypes] Improve shift through stack (PR #96151)

[via llvm-commits]

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@@ -238,18 +270,34 @@ define void @ashr_32bytes(ptr %src.ptr, ptr %byteOff.ptr, ptr %dst) nounwind {
 ; ALL-NEXT:    ldp x9, x8, [x0, #16]
 ; ALL-NEXT:    ldr x10, [x1]
 ; ALL-NEXT:    ldr q0, [x0]
-; ALL-NEXT:    and x10, x10, #0x1f
+; ALL-NEXT:    and x11, x10, #0x18
 ; ALL-NEXT:    stp x9, x8, [sp, #16]
 ; ALL-NEXT:    asr x8, x8, #63
 ; ALL-NEXT:    mov x9, sp
 ; ALL-NEXT:    str q0, [sp]
+; ALL-NEXT:    add x9, x9, x11
 ; ALL-NEXT:    stp x8, x8, [sp, #48]
 ; ALL-NEXT:    stp x8, x8, [sp, #32]
-; ALL-NEXT:    add x8, x9, x10
-; ALL-NEXT:    ldp x10, x9, [x8, #16]
-; ALL-NEXT:    ldr q0, [x8]
-; ALL-NEXT:    str q0, [x2]
-; ALL-NEXT:    stp x10, x9, [x2, #16]
+; ALL-NEXT:    lsl x8, x10, #3
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futog wrote:

Maybe we could add a logic the calculate the shifting unit based on the support of unaligned memory access. I was thinking something like this:
`
  Align LoadAlign = [&]() {
    EVT LoadVT = VT;
    do {
      LoadVT = TLI.getTypeToTransformTo(*DAG.getContext(), LoadVT);
    } while (!TLI.isTypeLegal(LoadVT));

    Align LoadAlign(LoadVT.getStoreSize());
    for (Align Tmp = LoadAlign;; Tmp = Tmp.previous()) {
      unsigned IsFast = 0;
      const bool AllowsFastMisalignedMemoryAccesses =
          TLI.allowsMisalignedMemoryAccesses(
              LoadVT, DAG.getDataLayout().getAllocaAddrSpace(), Tmp,
              MachineMemOperand::MOLoad, &IsFast) &&
          IsFast;
      if (not AllowsFastMisalignedMemoryAccesses) {
        break;
      }
      LoadAlign = Tmp;
      if (LoadAlign == Align(1))
        break;
    }
    return LoadAlign;
  }();
  const unsigned ShiftUnitInBits = LoadAlign.value() * 8;
  ...
  // And load it! While the load is not legal, legalizing it is obvious.
  SDValue Res =
      DAG.getLoad(VT, dl, Ch, AdjStackPtr,
                  MachinePointerInfo::getUnknownStack(DAG.getMachineFunction()),
                  commonAlignment(StackAlign, LoadAlign.value()));


`
@arsenm I know that you have already approved the PR, but what do you think?

https://github.com/llvm/llvm-project/pull/96151