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

[Prasoon Mishra via llvm-commits]

https://github.com/PrasoonMishra created https://github.com/llvm/llvm-project/pull/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:
- AU.addUsedIfAvailable<MachineLoopInfoWrapperPass>() to declare the optional dependency
- A test that verifies the optimization fires when MLI is accessible

>From ca10c400ff1b81aded5f29b35eb4c2f93cc54714 Mon Sep 17 00:00:00 2001
From: Prasoon Mishra <Prasoon.Mishra at amd.com>
Date: Tue, 28 Oct 2025 10:00:13 +0000
Subject: [PATCH] [PHIElimination] Declare MachineLoopInfo as optional
 dependency for legacy PM

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 AU.addUsedIfAvailable<MachineLoopInfoWrapperPass>() to
declare the optional dependency. Adds a test that verifies the
optimization fires when MLI is accessible.
---
 llvm/lib/CodeGen/PHIElimination.cpp           |  1 +
 .../CodeGen/AMDGPU/phi-elim-mli-available.mir | 44 +++++++++++++++++++
 2 files changed, 45 insertions(+)
 create mode 100644 llvm/test/CodeGen/AMDGPU/phi-elim-mli-available.mir

diff --git a/llvm/lib/CodeGen/PHIElimination.cpp b/llvm/lib/CodeGen/PHIElimination.cpp
index 34a9d5d0e401f..74e46121e65c7 100644
--- a/llvm/lib/CodeGen/PHIElimination.cpp
+++ b/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>();
diff --git a/llvm/test/CodeGen/AMDGPU/phi-elim-mli-available.mir b/llvm/test/CodeGen/AMDGPU/phi-elim-mli-available.mir
new file mode 100644
index 0000000000000..90315cae9d34c
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/phi-elim-mli-available.mir
@@ -0,0 +1,44 @@
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -run-pass=machine-loops,livevars,phi-node-elimination -no-phi-elim-live-out-early-exit -stats -filetype=null %s 2>&1 | FileCheck %s
+# REQUIRES: asserts
+
+# 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.
+#
+# CHECK: 2 phi-node-elimination - Number of critical edges split
+
+---
+name:            test_loop_exit_edge_split
+tracksRegLiveness: true
+body:             |
+  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
+...