[AMDGPU] LRO: allow same-BB non-lookthrough users for PHI #160909
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@llvm/pr-subscribers-backend-amdgpu Author: None (michaelselehov) ChangesLoop 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. Full diff: https://github.com/llvm/llvm-project/pull/160909.diff 1 Files Affected:
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))
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arsenm
reviewed
Sep 26, 2025
| @@ -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 | |||
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Added a minimal IR test (lro-phi-samebb-nonlookthrough-store.ll) that checks the same-BB filter looks at the user and allows non-lookthrough when the def is a 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.
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arsenm
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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). 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.
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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). 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.