[AArch64] Avoid pairing loads when the base reg is modified

When pairing loads, we should check if in between the two loads the
base register has been modified. If that is the case then avoid pairing
them because the second load actually loads from a different address.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D86956

Author: Congzhe Cao

llvm/lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp

@@ -1564,6 +1564,15 @@ AArch64LoadStoreOpt::findMatchingInsn(MachineBasicBlock::iterator I,
           continue;
         }
 
+        // If the BaseReg has been modified, then we cannot do the optimization.
+        // For example, in the following pattern
+        //   ldr x1 [x2]
+        //   ldr x2 [x3]
+        //   ldr x4 [x2, #8],
+        // the first and third ldr cannot be converted to ldp x1, x4, [x2]
+        if (!ModifiedRegUnits.available(BaseReg))
+          return E;
+
         // If the Rt of the second instruction was not modified or used between
         // the two instructions and none of the instructions between the second
         // and first alias with the second, we can combine the second into the

llvm/test/CodeGen/AArch64/aarch64-ldst-modified-baseReg.mir

@@ -0,0 +1,105 @@
+# RUN: llc -mtriple=aarch64-linux-gnu -verify-machineinstrs -run-pass=aarch64-ldst-opt %s -o - | FileCheck %s
+#
+# When the AArch64 Load Store Optimization pass tries to convert load instructions
+# into a ldp instruction, and when the base register of the second ldr instruction
+# has been modified in between these two ldr instructions, the conversion should not
+# occur.
+#
+# For example, for the following pattern:
+#     ldr x9 [x10]
+#     ldr x10 [x8]
+#     ldr x10 [x10, 8],
+# the first and third ldr instructions cannot be converted to ldp x9, x10, [x10].
+#
+# CHECK-LABEL: name: ldr-modified-baseReg-no-ldp1
+# CHECK-NOT: LDP
+# CHECK: $x9 = LDRXui $x10, 1 :: (load 8)
+# CHECK: $x10 = LDURXi $x8, 1 :: (load 8)
+# CHECK: $x10 = LDRXui $x10, 0 :: (load 8)
+# CHECK: RET
+---
+name: ldr-modified-baseReg-no-ldp1
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $x8, $x10
+
+    $x9 = LDRXui $x10, 1 :: (load 8)
+    $x10 = LDURXi $x8, 1 :: (load 8)
+    $x10 = LDRXui $x10, 0 :: (load 8)
+    RET undef $lr, implicit undef $w0
+...
+
+# CHECK-LABEL: name: str-modified-baseReg-no-stp1
+# CHECK-NOT: STP
+# CHECK: STRXui $x9, $x10, 1 :: (store 8)
+# CHECK: $x10 = LDRXui $x8, 0 :: (load 8)
+# CHECK: STRXui $x10, $x10, 0 :: (store 8)
+# CHECK: RET
+---
+name: str-modified-baseReg-no-stp1
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $x9, $x8, $x10
+
+    STRXui $x9, $x10, 1 :: (store 8)
+    $x10 = LDRXui $x8, 0 :: (load 8)
+    STRXui $x10, $x10, 0 :: (store 8)
+    RET undef $lr, implicit undef $w0
+...
+
+# CHECK-LABEL: name: ldr-modified-baseReg-no-ldp2
+# CHECK-NOT: LDP
+# CHECK: $x9 = LDRXui $x10, 1 :: (load 8)
+# CHECK: $x10 = MOVi64imm 13
+# CHECK: $x11 = LDRXui $x10, 0 :: (load 8)
+# CHECK: RET
+---
+name: ldr-modified-baseReg-no-ldp2
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $x8, $x10
+
+    $x9 = LDRXui $x10, 1 :: (load 8)
+    $x10 = MOVi64imm 13
+    $x11 = LDRXui $x10, 0 :: (load 8)
+    RET undef $lr, implicit undef $w0
+...
+
+# CHECK-LABEL: name: ldr-modified-baseReg-no-ldp3
+# CHECK-NOT: LDP
+# CHECK: $x9 = LDRXui $x10, 1 :: (load 8)
+# CHECK: $x10 = ADDXri $x8, $x11, 0
+# CHECK: $x12 = LDRXui $x10, 0 :: (load 8)
+# CHECK: RET
+---
+name: ldr-modified-baseReg-no-ldp3
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $x8, $x10, $x11
+
+    $x9 = LDRXui $x10, 1 :: (load 8)
+    $x10 = ADDXri $x8, $x11, 0
+    $x12 = LDRXui $x10, 0 :: (load 8)
+    RET undef $lr, implicit undef $w0
+...
+
+# CHECK-LABEL: name: ldr-modified-baseAddr-convert-to-ldp
+# CHECK: $x12, $x9 = LDPXi $x10, 0 :: (load 8)
+# CHECK: STRXui $x11, $x10, 1 :: (store 8)
+# CHECK: RET
+---
+name: ldr-modified-baseAddr-convert-to-ldp
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $x8, $x10, $x11
+
+    $x9 = LDRXui $x10, 1 :: (load 8)
+    STRXui $x11, $x10, 1 :: (store 8)
+    $x12 = LDRXui $x10, 0 :: (load 8)
+    RET undef $lr, implicit undef $w0
+...