[PHIElimination] Declare MachineLoopInfo dependency for Legacy PM (#169693)

PHIElimination uses MachineLoopInfo for loop-exiting critical edge
splitting but wasn't declaring this dependency via addUsedIfAvailable()
in getAnalysisUsage(). Without this declaration, the pass manager does
not make MachineLoopInfo accessible to PHIElimination, causing
getAnalysisIfAvailable() to return nullptr.

Without MachineLoopInfo, the loop-exiting edge optimization doesn't
fire, resulting in fewer critical edge splits and potentially suboptimal
code placement.

This patch adds:
- MachineLoopInfo as an optional dependency
- A test that verifies the optimization fires when MLI is accessible

Author: PrasoonMishra

llvm/lib/CodeGen/PHIElimination.cpp

@@ -201,6 +201,7 @@ INITIALIZE_PASS_END(PHIElimination, DEBUG_TYPE,
 
 void PHIElimination::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.addUsedIfAvailable<LiveVariablesWrapperPass>();
+  AU.addUsedIfAvailable<MachineLoopInfoWrapperPass>();
   AU.addPreserved<LiveVariablesWrapperPass>();
   AU.addPreserved<SlotIndexesWrapperPass>();
   AU.addPreserved<LiveIntervalsWrapperPass>();

llvm/test/CodeGen/AMDGPU/phi-elim-mli-available.mir

@@ -0,0 +1,85 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 6
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -run-pass=machine-loops,livevars,phi-node-elimination -no-phi-elim-live-out-early-exit -o - %s | FileCheck %s
+
+# Test that PHIElimination accesses MachineLoopInfo if available for
+# loop-aware edge splitting. The -no-phi-elim-live-out-early-exit flag
+# ensures we reach the loop exiting edge optimization code path.
+
+---
+name:            test_loop_exit_edge_split
+tracksRegLiveness: true
+body:             |
+  ; CHECK-LABEL: name: test_loop_exit_edge_split
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x80000000)
+  ; CHECK-NEXT:   liveins: $vgpr0, $sgpr0
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY:%[0-9]+]]:vgpr_32 = COPY killed $vgpr0
+  ; CHECK-NEXT:   [[COPY1:%[0-9]+]]:sgpr_32 = COPY killed $sgpr0
+  ; CHECK-NEXT:   [[V_MOV_B32_e32_:%[0-9]+]]:vgpr_32 = V_MOV_B32_e32 0, implicit $exec
+  ; CHECK-NEXT:   [[COPY2:%[0-9]+]]:vgpr_32 = COPY killed [[V_MOV_B32_e32_]]
+  ; CHECK-NEXT:   S_BRANCH %bb.1
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.4(0x40000000), %bb.2(0x40000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY3:%[0-9]+]]:vgpr_32 = COPY killed [[COPY2]]
+  ; CHECK-NEXT:   [[V_CMP_LT_I32_e64_:%[0-9]+]]:sreg_64_xexec = V_CMP_LT_I32_e64 [[COPY3]], [[COPY1]], implicit $exec
+  ; CHECK-NEXT:   $vcc = COPY killed [[V_CMP_LT_I32_e64_]]
+  ; CHECK-NEXT:   S_CBRANCH_VCCZ %bb.2, implicit killed $vcc
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.4:
+  ; CHECK-NEXT:   successors: %bb.3(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY4:%[0-9]+]]:vgpr_32 = COPY killed [[COPY3]]
+  ; CHECK-NEXT:   S_BRANCH %bb.3
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2:
+  ; CHECK-NEXT:   successors: %bb.5(0x40000000), %bb.1(0x40000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[V_ADD_U32_e64_:%[0-9]+]]:vgpr_32 = V_ADD_U32_e64 killed [[COPY3]], [[COPY]], 0, implicit $exec
+  ; CHECK-NEXT:   [[V_CMP_EQ_U32_e64_:%[0-9]+]]:sreg_64_xexec = V_CMP_EQ_U32_e64 [[V_ADD_U32_e64_]], [[COPY1]], implicit $exec
+  ; CHECK-NEXT:   $vcc = COPY killed [[V_CMP_EQ_U32_e64_]]
+  ; CHECK-NEXT:   [[COPY2:%[0-9]+]]:vgpr_32 = COPY [[V_ADD_U32_e64_]]
+  ; CHECK-NEXT:   S_CBRANCH_VCCZ %bb.1, implicit killed $vcc
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.5:
+  ; CHECK-NEXT:   successors: %bb.3(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   [[COPY4:%[0-9]+]]:vgpr_32 = COPY killed [[V_ADD_U32_e64_]]
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.3:
+  ; CHECK-NEXT:   [[COPY5:%[0-9]+]]:vgpr_32 = COPY killed [[COPY4]]
+  ; CHECK-NEXT:   $vgpr0 = COPY killed [[COPY5]]
+  ; CHECK-NEXT:   SI_RETURN implicit killed $vgpr0
+  bb.0:
+    liveins: $vgpr0, $sgpr0
+    %0:vgpr_32 = COPY $vgpr0
+    %1:sgpr_32 = COPY $sgpr0
+    %2:vgpr_32 = V_MOV_B32_e32 0, implicit $exec
+    S_BRANCH %bb.1
+
+  bb.1:
+    ; Loop header with PHI (multiple predecessors: bb.0, bb.2)
+    %3:vgpr_32 = PHI %2, %bb.0, %4, %bb.2
+
+    %10:sreg_64_xexec = V_CMP_LT_I32_e64 %3, %1, implicit $exec
+    $vcc = COPY %10
+    S_CBRANCH_VCCNZ %bb.3, implicit $vcc
+    S_BRANCH %bb.2
+
+  bb.2:
+    ; Loop body with two successors
+    %4:vgpr_32 = V_ADD_U32_e64 %3, %0, 0, implicit $exec
+
+    %11:sreg_64_xexec = V_CMP_EQ_U32_e64 %4, %1, implicit $exec
+    $vcc = COPY %11
+    S_CBRANCH_VCCNZ %bb.3, implicit $vcc
+    S_BRANCH %bb.1
+
+  bb.3:
+    ; Exit - PHI with values from both bb.1 and bb.2
+    %5:vgpr_32 = PHI %3, %bb.1, %4, %bb.2
+    $vgpr0 = COPY %5
+    SI_RETURN implicit $vgpr0
+...