[PowerPC] Add frame index alignment check if the the addressing mode is DS/DQ-Form, and fall back to X-Form if necessary.

[amyk]

Summary

This patch adds a function that checks whether or not the frame index
is aligned when the computed addressing mode is an aligned D-Form (DS, or DQ-Form).
If the frame index appears to be unaligned, within these two modes, reset
the mode to X-Form in order to fall back to selection X-Form loads.

A test case is added to ensure that the test emits X-Form loads and not DQ-Form
loads since the frame index is not aligned within the test case.

For this patch, lxvx is produced since it's pattern utilizes ForceXForm, but this is
to ensure that the follow up patch, D95115, will continue to produce lxvx when the instruction is
updated to no longer use ForceXForm (and instead just use XForm).

[nemanjai]

My comments are stylistic so I don't have a problem with you addressing it on the commit without another review. LGTM.

llvm/lib/Target/PowerPC/PPCISelLowering.cpp

17338

This is kind of minor, but when the whole function is inside a conditional block, it should probably just be an early return. You also don't really need a dyn_cast there - as far as I can tell you don't use FI. Probably a simple

if (!isa<FrameIndexSDNode>(N))
  return;

should suffice.

17339–17341

This can just be an or of the two conditions.

[amyk]

Update revision to reflect the suggestions made by Nemanja.

llvm/lib/Target/PowerPC/PPCISelLowering.cpp

PPC::AddrMode PPCTargetLowering::SelectForceXFormMode(SDValue N, SDValue &Disp,
   // Otherwise, use R0 as the base register.
   Disp = DAG.getRegister(Subtarget.isPPC64() ? PPC::ZERO8 : PPC::ZERO,
                          N.getValueType());
   Base = N;
 
   return Mode;
 }
 
+// If we happen to match to an aligned D-Form, check if the Frame Index is
+// adequately aligned. If it is not, reset the mode to match to X-Form.
+static void setXFormForUnalignedFI(SDValue N, unsigned Flags,
+                                   PPC::AddrMode &Mode) {
+  if (FrameIndexSDNode *FI = dyn_cast<FrameIndexSDNode>(N)) {
+    if (Mode == PPC::AM_DSForm && !(Flags & PPC::MOF_RPlusSImm16Mult4))
+      Mode = PPC::AM_XForm;
+    if (Mode == PPC::AM_DQForm && !(Flags & PPC::MOF_RPlusSImm16Mult16))
+      Mode = PPC::AM_XForm;
+  }
+}
+
 /// SelectOptimalAddrMode - Based on a node N and it's Parent (a MemSDNode),
 /// compute the address flags of the node, get the optimal address mode based
 /// on the flags, and set the Base and Disp based on the address mode.
 PPC::AddrMode PPCTargetLowering::SelectOptimalAddrMode(const SDNode *Parent,
                                                        SDValue N, SDValue &Disp,
                                                        SDValue &Base,
                                                        SelectionDAG &DAG,
                                                        MaybeAlign Align) const {
   SDLoc DL(Parent);
 
   // Compute the address flags.
   unsigned Flags = computeMOFlags(Parent, N, DAG);
 
   // Get the optimal address mode based on the Flags.
   PPC::AddrMode Mode = getAddrModeForFlags(Flags);
 
+  // If the address mode is DS-Form or DQ-Form, check if the FI is aligned.
+  // Select an X-Form load if it is not.
+  setXFormForUnalignedFI(N, Flags, Mode);
+
   // Set Base and Disp accordingly depending on the address mode.
   switch (Mode) {
   case PPC::AM_DForm:
   case PPC::AM_DSForm:
   case PPC::AM_DQForm: {
     // This is a register plus a 16-bit immediate. The base will be the
     // register and the displacement will be the immediate unless it
     // isn't sufficiently aligned.

llvm/test/CodeGen/PowerPC/unaligned-dqform-ld.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -verify-machineinstrs -mtriple=powerpc64le-unknown-linux-gnu \
+; RUN:   -mcpu=pwr9 -ppc-asm-full-reg-names -ppc-vsr-nums-as-vr < %s | \
+; RUN:   FileCheck %s --check-prefix=CHECK-P9-LE
+; RUN: llc -verify-machineinstrs -mtriple=powerpc64-unknown-linux-gnu \
+; RUN:   -mcpu=pwr9 -ppc-asm-full-reg-names -ppc-vsr-nums-as-vr < %s | \
+; RUN:   FileCheck %s --check-prefix=CHECK-P9-BE
+
+%0 = type { %1 }
+%1 = type { %2, %6 }
+%2 = type { %3 }
+%3 = type { %4 }
+%4 = type { %5 }
+%5 = type { i8*, i32, i32 }
+%6 = type { [160 x i8] }
+%7 = type { %8, i32, %8 }
+%8 = type { i8*, i64 }
+
+$abc = comdat any
+
+; This test checks that X-Form load, lxvx, is being produced here instead of
+; the DQ-Form, lxv. We should not be producing lxv here as the frame index
+; value is unaligned (not a multiple of 16).
+define void @abc(%0* %arg, [5 x i64] %arg1, [5 x i64] %arg2, [5 x i64] %arg3, [5 x i64] %arg4) local_unnamed_addr #0 comdat {
+; CHECK-P9-LE-LABEL: abc:
+; CHECK-P9-LE:       # %bb.0: # %bb
+; CHECK-P9-LE-NEXT:    addi r6, r1, 120
+; CHECK-P9-LE-NEXT:    addi r11, r1, 104
+; CHECK-P9-LE-NEXT:    ld r0, 136(r1)
+; CHECK-P9-LE-NEXT:    ld r12, 144(r1)
+; CHECK-P9-LE-NEXT:    std r4, 0(r3)
+; CHECK-P9-LE-NEXT:    addi r4, r1, -160
+; CHECK-P9-LE-NEXT:    lxvx vs0, 0, r6
+; CHECK-P9-LE-NEXT:    lxvx vs1, 0, r11
+; CHECK-P9-LE-NEXT:    ld r6, 192(r1)
+; CHECK-P9-LE-NEXT:    ld r11, 160(r1)
+; CHECK-P9-LE-NEXT:    std r8, 0(r3)
+; CHECK-P9-LE-NEXT:    std r10, 0(r3)
+; CHECK-P9-LE-NEXT:    stxv vs1, 64(r4)
+; CHECK-P9-LE-NEXT:    std r0, 0(r3)
+; CHECK-P9-LE-NEXT:    std r11, 0(r3)
+; CHECK-P9-LE-NEXT:    std r6, 0(r3)
+; CHECK-P9-LE-NEXT:    addi r3, r3, 16
+; CHECK-P9-LE-NEXT:    std r5, 0(0)
+; CHECK-P9-LE-NEXT:    std r7, -136(r1)
+; CHECK-P9-LE-NEXT:    std r9, 0(0)
+; CHECK-P9-LE-NEXT:    stxv vs0, 80(r4)
+; CHECK-P9-LE-NEXT:    std r12, 0(0)
+; CHECK-P9-LE-NEXT:    std r3, 0(r3)
+; CHECK-P9-LE-NEXT:    blr
+;
+; CHECK-P9-BE-LABEL: abc:
+; CHECK-P9-BE:       # %bb.0: # %bb
+; CHECK-P9-BE-NEXT:    addi r6, r1, 136
+; CHECK-P9-BE-NEXT:    addi r11, r1, 120
+; CHECK-P9-BE-NEXT:    ld r0, 152(r1)
+; CHECK-P9-BE-NEXT:    ld r12, 160(r1)
+; CHECK-P9-BE-NEXT:    std r4, 0(r3)
+; CHECK-P9-BE-NEXT:    addi r4, r1, -160
+; CHECK-P9-BE-NEXT:    lxvx vs0, 0, r6
+; CHECK-P9-BE-NEXT:    lxvx vs1, 0, r11
+; CHECK-P9-BE-NEXT:    ld r6, 208(r1)
+; CHECK-P9-BE-NEXT:    ld r11, 176(r1)
+; CHECK-P9-BE-NEXT:    std r8, 0(r3)
+; CHECK-P9-BE-NEXT:    std r10, 0(r3)
+; CHECK-P9-BE-NEXT:    stxv vs1, 64(r4)
+; CHECK-P9-BE-NEXT:    std r0, 0(r3)
+; CHECK-P9-BE-NEXT:    std r11, 0(r3)
+; CHECK-P9-BE-NEXT:    std r6, 0(r3)
+; CHECK-P9-BE-NEXT:    addi r3, r3, 16
+; CHECK-P9-BE-NEXT:    std r5, 0(0)
+; CHECK-P9-BE-NEXT:    std r7, -136(r1)
+; CHECK-P9-BE-NEXT:    std r9, 0(0)
+; CHECK-P9-BE-NEXT:    stxv vs0, 80(r4)
+; CHECK-P9-BE-NEXT:    std r12, 0(0)
+; CHECK-P9-BE-NEXT:    std r3, 0(r3)
+; CHECK-P9-BE-NEXT:    blr
+bb:
+  %i = alloca [4 x %7], align 8
+  %i5 = extractvalue [5 x i64] %arg1, 0
+  %i6 = extractvalue [5 x i64] %arg1, 1
+  %i7 = extractvalue [5 x i64] %arg1, 3
+  %i8 = extractvalue [5 x i64] %arg1, 4
+  %i9 = extractvalue [5 x i64] %arg2, 0
+  %i10 = extractvalue [5 x i64] %arg2, 1
+  %i11 = extractvalue [5 x i64] %arg2, 3
+  %i12 = extractvalue [5 x i64] %arg2, 4
+  %i13 = extractvalue [5 x i64] %arg3, 0
+  %i14 = extractvalue [5 x i64] %arg3, 1
+  %i15 = extractvalue [5 x i64] %arg3, 2
+  %i16 = extractvalue [5 x i64] %arg3, 3
+  %i17 = extractvalue [5 x i64] %arg4, 0
+  %i18 = extractvalue [5 x i64] %arg4, 4
+  store i64 %i5, i64* undef, align 8
+  store i64 %i6, i64* null, align 8
+  %i19 = getelementptr inbounds [4 x %7], [4 x %7]* %i, i64 0, i64 0, i32 2
+  %i20 = bitcast %8* %i19 to i64*
+  store i64 %i7, i64* %i20, align 8
+  store i64 %i8, i64* undef, align 8
+  store i64 %i9, i64* null, align 8
+  store i64 %i10, i64* undef, align 8
+  %i21 = getelementptr inbounds [4 x %7], [4 x %7]* %i, i64 0, i64 1, i32 2
+  %i22 = bitcast %8* %i21 to i64*
+  store i64 %i11, i64* %i22, align 8
+  %i23 = getelementptr inbounds [4 x %7], [4 x %7]* %i, i64 0, i64 1, i32 2, i32 1
+  store i64 %i12, i64* %i23, align 8
+  %i24 = getelementptr inbounds [4 x %7], [4 x %7]* %i, i64 0, i64 2
+  %i25 = bitcast %7* %i24 to i64*
+  store i64 %i13, i64* %i25, align 8
+  %i26 = getelementptr inbounds [4 x %7], [4 x %7]* %i, i64 0, i64 2, i32 0, i32 1
+  store i64 %i14, i64* %i26, align 8
+  store i64 %i15, i64* undef, align 8
+  store i64 %i16, i64* null, align 8
+  store i64 %i17, i64* undef, align 8
+  store i64 undef, i64* null, align 8
+  store i64 %i18, i64* undef, align 8
+  %i27 = getelementptr inbounds %0, %0* %arg, i64 0, i32 0, i32 0, i32 0
+  %i28 = getelementptr inbounds %3, %3* %i27, i64 1, i32 0
+  store %4* %i28, %4** undef, align 8
+  ret void
+}