[AArch64] Fix for the pre-indexed paired load/store optimization.

This patch fixes an issue where a pre-indexed store e.g.,
STR x1, [x0, rust-lang#24]! with a store like STR x0, [x0, rust-lang#8] are
merged into a single store: STP x1, x0, [x0, rust-lang#24]!
. They shouldn’t be merged because the second store uses
x0 as both the stored value and the address and so it needs to be using the updated x0.
Therefore, it should not be folded into a STP <>pre.

Additionally a new test case is added to verify this fix.

Differential Revision: https://reviews.llvm.org/D101888

Change-Id: I26f1985ac84e970961e2cdca23c590fa6773851a

llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp

@@ -1610,7 +1610,13 @@ AArch64LoadStoreOpt::findMatchingInsn(MachineBasicBlock::iterator I,
               !UsedRegUnits.available(getLdStBaseOp(MI).getReg());
           bool IsBaseRegModified =
               !ModifiedRegUnits.available(getLdStBaseOp(MI).getReg());
-          if (IsOutOfBounds || IsBaseRegUsed || IsBaseRegModified) {
+          // If the stored value and the address of the second instruction is
+          // the same, it needs to be using the updated register and therefore
+          // it must not be folded.
+          bool IsMIRegTheSame =
+              getLdStRegOp(MI).getReg() == getLdStBaseOp(MI).getReg();
+          if (IsOutOfBounds || IsBaseRegUsed || IsBaseRegModified ||
+              IsMIRegTheSame) {
             LiveRegUnits::accumulateUsedDefed(MI, ModifiedRegUnits,
                                               UsedRegUnits, TRI);
             MemInsns.push_back(&MI);

llvm/test/CodeGen/AArch64/strpre-str-merge.mir

@@ -424,3 +424,30 @@ body:             |
     STRSui killed renamable $s1, renamable $x0, 1 :: (store 4)
     RET undef $lr, implicit $x0
 ...
+
+---
+name:            16-strxpre-strxui-same-reg-no-merge
+alignment:       4
+tracksRegLiveness: true
+liveins:
+  - { reg: '$x0' }
+  - { reg: '$x1' }
+  - { reg: '$x2' }
+frameInfo:
+  maxAlignment:    1
+  maxCallFrameSize: 0
+machineFunctionInfo:
+  hasRedZone:      false
+body:             |
+  bb.0.entry:
+    liveins: $x0, $x1, $x2
+    ; CHECK-LABEL: name: 16-strxpre-strxui-same-reg-no-merge
+    ; CHECK: liveins: $x0, $x1, $x2
+    ; CHECK: early-clobber renamable $x0 = STRXpre renamable $x1, renamable $x0, 24, implicit $w0 :: (store 8)
+    ; CHECK: STRXui renamable $x0, renamable $x0, 1 :: (store 8)
+    ; CHECK: RET undef $lr, implicit $x0
+    early-clobber renamable $x0 = STRXpre killed renamable $x1, killed renamable $x0, 24 :: (store 8)
+    STRXui renamable $x0, renamable $x0, 1 :: (store 8)
+    RET undef $lr, implicit $x0
+
+...