[MachineLICM] Do not rely on hasSideEffect when handling impdefs

[atrosinenko]

Take clobbered registers described as implicit-def operands into
account when deciding if an instruction can be moved.

When hasSideEffect was set to 0 in the definition of LOADgotAUTH,
MultiSource/Benchmarks/Ptrdist/ks/ks test from llvm-test-suite started
to crash. The issue was traced down to MachineLICM pass placing
LOADgotAUTH right after an unrelated copy to x16 like rewriting this
code:

bb.0:
  renamable $x16 = COPY renamable $x12
  B %bb.1

bb.1:
  ...
  /* use $x16 */
  ...
  renamable $x20 = LOADgotAUTH target-flags(aarch64-got) @some_variable, implicit-def dead $x16, implicit-def dead $x17, implicit-def dead $nzcv
  /* use $x20 */
  ...

like the following:

bb.0:
  renamable $x16 = COPY renamable $x12
  renamable $x20 = LOADgotAUTH target-flags(aarch64-got) @some_variable, implicit-def dead $x16, implicit-def dead $x17, implicit-def dead $nzcv
  B %bb.1

bb.1:
  ...
  /* use $x16 */
  ...
  /* use $x20 */
  ...

[atrosinenko]

Relates to #130809, #141330.

[llvmbot]

@llvm/pr-subscribers-backend-aarch64

Author: Anatoly Trosinenko (atrosinenko)

Changes

Take clobbered registers described as implicit-def operands into
account when deciding if an instruction can be moved.

When hasSideEffect was set to 0 in the definition of LOADgotAUTH,
MultiSource/Benchmarks/Ptrdist/ks/ks test from llvm-test-suite started
to crash. The issue was traced down to MachineLICM pass placing
LOADgotAUTH right after an unrelated copy to x16 like rewriting this
code:

bb.0:
  renamable $x16 = COPY renamable $x12
  B %bb.1

bb.1:
  ...
  /* use $x16 */
  ...
  renamable $x20 = LOADgotAUTH target-flags(aarch64-got) @<!-- -->some_variable, implicit-def dead $x16, implicit-def dead $x17, implicit-def dead $nzcv
  /* use $x20 */
  ...

like the following:

bb.0:
  renamable $x16 = COPY renamable $x12
  renamable $x20 = LOADgotAUTH target-flags(aarch64-got) @<!-- -->some_variable, implicit-def dead $x16, implicit-def dead $x17, implicit-def dead $nzcv
  B %bb.1

bb.1:
  ...
  /* use $x16 */
  ...
  /* use $x20 */
  ...

---

Full diff: https://github.com/llvm/llvm-project/pull/145379.diff

2 Files Affected:

• (modified) llvm/lib/CodeGen/MachineLICM.cpp (+7-10)
• (added) llvm/test/CodeGen/AArch64/mlicm-implicit-defs.mir (+53)

diff --git a/llvm/lib/CodeGen/MachineLICM.cpp b/llvm/lib/CodeGen/MachineLICM.cpp
index c9079170ca575..5841a9ffa7a99 100644
--- a/llvm/lib/CodeGen/MachineLICM.cpp
+++ b/llvm/lib/CodeGen/MachineLICM.cpp
@@ -559,18 +559,15 @@ void MachineLICMImpl::ProcessMI(MachineInstr *MI, BitVector &RUDefs,
       if (!MO.isDead())
         // Non-dead implicit def? This cannot be hoisted.
         RuledOut = true;
-      // No need to check if a dead implicit def is also defined by
-      // another instruction.
-      continue;
+    } else {
+      // FIXME: For now, avoid instructions with multiple defs, unless
+      // it's a dead implicit def.
+      if (Def)
+        RuledOut = true;
+      else
+        Def = Reg;
     }
 
-    // FIXME: For now, avoid instructions with multiple defs, unless
-    // it's a dead implicit def.
-    if (Def)
-      RuledOut = true;
-    else
-      Def = Reg;
-
     // If we have already seen another instruction that defines the same
     // register, then this is not safe.  Two defs is indicated by setting a
     // PhysRegClobbers bit.
diff --git a/llvm/test/CodeGen/AArch64/mlicm-implicit-defs.mir b/llvm/test/CodeGen/AArch64/mlicm-implicit-defs.mir
new file mode 100644
index 0000000000000..51cb35116dc62
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/mlicm-implicit-defs.mir
@@ -0,0 +1,53 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
+# RUN: llc -mtriple=aarch64 -run-pass machinelicm -verify-machineinstrs -o - %s | FileCheck %s
+# RUN: llc -mtriple=aarch64 -passes machinelicm -o - %s | FileCheck %s
+
+---
+name: test
+tracksRegLiveness: true
+body: |
+  ; CHECK-LABEL: name: test
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT:   liveins: $x0
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   $x16 = COPY killed $x0
+  ; CHECK-NEXT:   B %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.1(0x40000000), %bb.2(0x40000000)
+  ; CHECK-NEXT:   liveins: $x16
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   $x1 = COPY killed $x16
+  ; CHECK-NEXT:   $x2 = MOVi64imm 1024, implicit-def dead $x16
+  ; CHECK-NEXT:   $x16 = LDRXroX killed $x1, killed $x2, 0, 0
+  ; CHECK-NEXT:   $xzr = SUBSXri $x16, 0, 0, implicit-def $nzcv
+  ; CHECK-NEXT:   Bcc 1, %bb.1, implicit $nzcv
+  ; CHECK-NEXT:   B %bb.2
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2:
+  ; CHECK-NEXT:   liveins: $x1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   $x0 = COPY killed $x1
+  ; CHECK-NEXT:   RET_ReallyLR
+  bb.0:
+    liveins: $x0
+    $x16 = COPY killed $x0
+    B %bb.1
+
+  bb.1:
+    liveins: $x16
+    $x1 = COPY killed $x16
+    /* MOVi64imm below mimics a pseudo instruction that doesn't have any */
+    /* unmodelled side effects, but uses x16 as a scratch register.      */
+    $x2 = MOVi64imm 1024, implicit-def dead $x16
+    $x16 = LDRXroX killed $x1, killed $x2, 0, 0
+    $xzr = SUBSXri $x16, 0, 0, implicit-def $nzcv
+    Bcc 1, %bb.1, implicit $nzcv
+    B %bb.2
+
+  bb.2:
+    liveins: $x1
+    $x0 = COPY killed $x1
+    RET_ReallyLR
+...

[atrosinenko]

Fixed the test failures: the original fix ruled out safe-to-move instructions with dead implicit-def operands. Fixed this in 079d654 and updated precommitted test cases accordingly.