[llvm] [LV] Emit better debug and opt-report messages when vectorization is disallowed in the LoopVectorizer (PR #158513)

[Tibor Győri via llvm-commits]

https://github.com/TiborGY updated https://github.com/llvm/llvm-project/pull/158513

>From 81f1800118aab3462dbbd03023c2b052f7d455df Mon Sep 17 00:00:00 2001
From: GYT <tiborgyri at gmail.com>
Date: Sun, 14 Sep 2025 23:19:37 +0200
Subject: [PATCH 1/2] Emit better debug and opt-report messages when
 vectorization is disallowed in the LoopVectorizer

formatting

formatting2

fix ;) vs. ); typos

formatting3
---
 .../Vectorize/LoopVectorizationLegality.cpp   | 35 ++++++++++++++++---
 1 file changed, 31 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp
index ff35db14f7094..9010ca62aeee7 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp
@@ -182,14 +182,41 @@ void LoopVectorizeHints::setAlreadyVectorized() {
 bool LoopVectorizeHints::allowVectorization(
     Function *F, Loop *L, bool VectorizeOnlyWhenForced) const {
   if (getForce() == LoopVectorizeHints::FK_Disabled) {
-    LLVM_DEBUG(dbgs() << "LV: Not vectorizing: #pragma vectorize disable.\n");
-    emitRemarkWithHints();
+    if (Force.Value == LoopVectorizeHints::FK_Disabled) {
+      LLVM_DEBUG(dbgs() << "LV: Not vectorizing: #pragma vectorize disable.\n");
+      ORE.emit(OptimizationRemarkMissed(LV_NAME, "MissedExplicitlyDisabled",
+                                        TheLoop->getStartLoc(),
+                                        TheLoop->getHeader())
+               << "loop not vectorized: vectorization is explicitly disabled");
+    } else if (hasDisableAllTransformsHint(TheLoop)) {
+      LLVM_DEBUG(
+          dbgs() << "LV: Not vectorizing: loop hasDisableAllTransformsHint.\n");
+      ORE.emit(OptimizationRemarkMissed(LV_NAME, "MissedTrafoDisabled",
+                                        TheLoop->getStartLoc(),
+                                        TheLoop->getHeader())
+               << "loop not vectorized: loop transformations are disabled");
+    } else {
+      // This should be unreachable unless there is a bug.
+      LLVM_DEBUG(
+          dbgs() << "LV: [FIXME] Not vectorizing: loop vect disabled for "
+                    "an unknown reason!\n");
+      ORE.emit(OptimizationRemarkMissed(LV_NAME, "MissedUnknown",
+                                        TheLoop->getStartLoc(),
+                                        TheLoop->getHeader())
+               << "loop not vectorized: unknown reason, please file a bug "
+                  "report on the LLVM issue tracker");
+    }
     return false;
   }
 
   if (VectorizeOnlyWhenForced && getForce() != LoopVectorizeHints::FK_Enabled) {
-    LLVM_DEBUG(dbgs() << "LV: Not vectorizing: No #pragma vectorize enable.\n");
-    emitRemarkWithHints();
+    LLVM_DEBUG(dbgs() << "LV: Not vectorizing: VectorizeOnlyWhenForced is set, "
+                         "and no #pragma vectorize enable.\n");
+    ORE.emit(OptimizationRemarkMissed(LV_NAME, "MissedForceOnly",
+                                      TheLoop->getStartLoc(),
+                                      TheLoop->getHeader())
+             << "loop not vectorized: only vectorizing loops that "
+                "explicitly request it");
     return false;
   }
 

>From 6d941ff51d221e03a9d575086f4cec5efdf43dd2 Mon Sep 17 00:00:00 2001
From: GYT <tiborgyri at gmail.com>
Date: Mon, 15 Sep 2025 12:22:19 +0200
Subject: [PATCH 2/2] Trafo -> Transforms

---
 llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp
index 9010ca62aeee7..2dbab7d485ec0 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp
@@ -191,7 +191,7 @@ bool LoopVectorizeHints::allowVectorization(
     } else if (hasDisableAllTransformsHint(TheLoop)) {
       LLVM_DEBUG(
           dbgs() << "LV: Not vectorizing: loop hasDisableAllTransformsHint.\n");
-      ORE.emit(OptimizationRemarkMissed(LV_NAME, "MissedTrafoDisabled",
+      ORE.emit(OptimizationRemarkMissed(LV_NAME, "MissedTransformsDisabled",
                                         TheLoop->getStartLoc(),
                                         TheLoop->getHeader())
                << "loop not vectorized: loop transformations are disabled");