[llvm] [AMDGPU] LRO: allow same-BB non-lookthrough users for PHI (PR #160909)

[via llvm-commits]

https://github.com/michaelselehov created https://github.com/llvm/llvm-project/pull/160909

Loop headers frequently consume the loop-carried value in the header block via non-lookthrough ops (e.g. byte-wise vector binops). LiveRegOptimizer’s same-BB filter currently prunes these users, so the loop-carried PHI is not coerced to i32 and the intended packed form is lost.

Relax the filter: when the def is a PHI, allow same-BB non-lookthrough users. Also fix the check to look at the user (CII) rather than the def (II) so the walk does not terminate prematurely.

>From 8bd213db197ee0fa5ec1034274df86be6985d4cd Mon Sep 17 00:00:00 2001
From: Michael Selehov <michael.selehov at amd.com>
Date: Fri, 26 Sep 2025 10:10:54 -0500
Subject: [PATCH] [AMDGPU] LRO: allow same-BB non-lookthrough users for PHI
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Loop headers frequently consume the loop-carried value in the header
block via non-lookthrough ops (e.g. byte-wise vector binops). LRO’s
same-BB filter currently prunes these users, so the loop-carried PHI
is not coerced to i32 and the intended packed form is lost.

Relax the filter: when the def is a PHI, allow same-BB non-lookthrough
users. Also fix the check to look at the user (CII) rather than the
def (II) so the walk does not terminate prematurely.
---
 llvm/lib/Target/AMDGPU/AMDGPULateCodeGenPrepare.cpp | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Target/AMDGPU/AMDGPULateCodeGenPrepare.cpp b/llvm/lib/Target/AMDGPU/AMDGPULateCodeGenPrepare.cpp
index 38718c43a61dd..7504f1a8cea09 100644
--- a/llvm/lib/Target/AMDGPU/AMDGPULateCodeGenPrepare.cpp
+++ b/llvm/lib/Target/AMDGPU/AMDGPULateCodeGenPrepare.cpp
@@ -150,7 +150,10 @@ class LiveRegOptimizer {
       if (!CVisited.insert(CII).second)
         continue;
 
-      if (CII->getParent() == II->getParent() && !IsLookThru(II))
+      // Same-BB filter must look at the *user*; and allow non-lookthrough
+      // users when the def is a PHI (loop-header pattern).
+      if (CII->getParent() == II->getParent() && !IsLookThru(CII) &&
+          !isa<PHINode>(II))
         continue;
 
       if (isOpLegal(CII))