[clang] [clang] Also disable scalable vectorisation for vectorize(disable) (PR #227254)

[Gaëtan Bossu via cfe-commits]

https://github.com/gbossu created https://github.com/llvm/llvm-project/pull/227254

This ensures that #pragma clang loop vectorize(disable) also disables scalable vectorisation. Only setting the width to 1 could potentially enable LoopVectorizer to vectorize with VF = vscale x 1. Especially with the -scalable-vectorization=preferred option.

>From c4cc8fa0c3bc8f179507628f26a8e41928deae95 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ga=C3=ABtan=20Bossu?= <gaetan.bossu at arm.com>
Date: Tue, 29 Sep 2026 09:50:48 +0000
Subject: [PATCH] [clang] Also disable scalable vectorisation for
 vectorize(disable)

This ensures that #pragma clang loop vectorize(disable) also disables
scalable vectorisation. Only setting the width to 1 could potentially
enable LoopVectorizer to vectorize with VF = vscale x 1. Especially with
the -scalable-vectorization=preferred option.
---
 clang/lib/CodeGen/CGLoopInfo.cpp                |  5 +++--
 clang/test/CodeGenCXX/pragma-loop-predicate.cpp | 12 ++++++------
 clang/test/CodeGenCXX/pragma-loop-safety.cpp    |  3 ++-
 clang/test/CodeGenCXX/pragma-loop.cpp           |  6 +++---
 4 files changed, 14 insertions(+), 12 deletions(-)

diff --git a/clang/lib/CodeGen/CGLoopInfo.cpp b/clang/lib/CodeGen/CGLoopInfo.cpp
index c13c2f6c60089..14e13af69b011 100644
--- a/clang/lib/CodeGen/CGLoopInfo.cpp
+++ b/clang/lib/CodeGen/CGLoopInfo.cpp
@@ -651,9 +651,10 @@ void LoopInfoStack::push(BasicBlock *Header, clang::ASTContext &Ctx,
     case LoopHintAttr::Disable:
       switch (Option) {
       case LoopHintAttr::Vectorize:
-        // Disable vectorization by specifying a width of 1.
+        // Disable vectorization by specifying a width of 1 and disabling
+        // scalable vectorization explicitly.
         setVectorizeWidth(1);
-        setVectorizeScalable(LoopAttributes::Unspecified);
+        setVectorizeScalable(LoopAttributes::Disable);
         break;
       case LoopHintAttr::Interleave:
         // Disable interleaving by speciyfing a count of 1.
diff --git a/clang/test/CodeGenCXX/pragma-loop-predicate.cpp b/clang/test/CodeGenCXX/pragma-loop-predicate.cpp
index bfacddc207f26..fdb23329e6121 100644
--- a/clang/test/CodeGenCXX/pragma-loop-predicate.cpp
+++ b/clang/test/CodeGenCXX/pragma-loop-predicate.cpp
@@ -37,8 +37,8 @@ void test3(int *List, int Length) {
     List[i] = i * 2;
 }
 
-// Check that disabling vectorization means a vectorization width of 1, and
-// also that vectorization_predicate isn't enabled.
+// Check that disabling vectorization sets a width of 1 and disables
+// scalable vectorization without enabling vectorization_predicate.
 void test4(int *List, int Length) {
 // CHECK-LABEL: @{{.*}}test4{{.*}}(
 // CHECK: br label {{.*}}, !llvm.loop ![[LOOP4:.*]]
@@ -113,13 +113,13 @@ void test9(int *List, int Length) {
 
 // CHECK-NEXT: ![[LOOP3]] = distinct !{![[LOOP3]], [[MP]], [[GEN6]], [[GEN3]]}
 
-// CHECK-NEXT: ![[LOOP4]] = distinct !{![[LOOP4]], [[MP]], [[GEN10:![0-9]+]]}
+// CHECK-NEXT: ![[LOOP4]] = distinct !{![[LOOP4]], [[MP]], [[GEN10:![0-9]+]], [[GEN11:![0-9]+]]}
 // CHECK-NEXT: [[GEN10]] = !{!"llvm.loop.vectorize.width", i32 1}
+// CHECK-NEXT: [[GEN11]] = !{!"llvm.loop.vectorize.scalable.disable"}
 
-// CHECK-NEXT: ![[LOOP5]] = distinct !{![[LOOP5]], [[MP]], [[GEN6]], [[GEN10]]}
+// CHECK-NEXT: ![[LOOP5]] = distinct !{![[LOOP5]], [[MP]], [[GEN6]], [[GEN10]], [[GEN11]]}
 
-// CHECK-NEXT: ![[LOOP6]] = distinct !{![[LOOP6]], [[MP]], [[GEN8]], [[GEN10]], [[GEN11:![0-9]+]]}
-// CHECK-NEXT: [[GEN11]] = !{!"llvm.loop.vectorize.scalable.disable"}
+// CHECK-NEXT: ![[LOOP6]] = distinct !{![[LOOP6]], [[MP]], [[GEN8]], [[GEN10]], [[GEN11]]}
 
 // CHECK-NEXT: ![[LOOP7]] = distinct !{![[LOOP7]], [[MP]], [[GEN8]], [[GEN12:![0-9]+]], [[GEN11]], [[GEN3]]}
 // CHECK-NEXT: [[GEN12]] = !{!"llvm.loop.vectorize.width", i32 4}
diff --git a/clang/test/CodeGenCXX/pragma-loop-safety.cpp b/clang/test/CodeGenCXX/pragma-loop-safety.cpp
index e2a7a124890a6..2114eb1643190 100644
--- a/clang/test/CodeGenCXX/pragma-loop-safety.cpp
+++ b/clang/test/CodeGenCXX/pragma-loop-safety.cpp
@@ -53,6 +53,7 @@ void interleave_test(int *List, int Length) {
 // CHECK: ![[INTERLEAVE_1]] = !{!"llvm.loop.interleave.count", i32 1}
 // CHECK: ![[INTENABLE_1]] = !{!"llvm.loop.vectorize.enable"}
 // CHECK: ![[ACCESS_GROUP_8]] = distinct !{}
-// CHECK: ![[LOOP2_HINTS]] = distinct !{![[LOOP2_HINTS]], [[MP]], ![[PARALLEL_ACCESSES_11:[0-9]+]], ![[UNROLL_DISABLE]], ![[WIDTH_1:[0-9]+]], ![[INTENABLE_1]]}
+// CHECK: ![[LOOP2_HINTS]] = distinct !{![[LOOP2_HINTS]], [[MP]], ![[PARALLEL_ACCESSES_11:[0-9]+]], ![[UNROLL_DISABLE]], ![[WIDTH_1:[0-9]+]], ![[FIXED_VEC:[0-9]+]], ![[INTENABLE_1]]}
 // CHECK: ![[PARALLEL_ACCESSES_11]] = !{!"llvm.loop.parallel_accesses", ![[ACCESS_GROUP_8]]}
 // CHECK: ![[WIDTH_1]] = !{!"llvm.loop.vectorize.width", i32 1}
+// CHECK: ![[FIXED_VEC]] = !{!"llvm.loop.vectorize.scalable.disable"}
diff --git a/clang/test/CodeGenCXX/pragma-loop.cpp b/clang/test/CodeGenCXX/pragma-loop.cpp
index 75961aa86dcad..dbec1e3a1f5b1 100644
--- a/clang/test/CodeGenCXX/pragma-loop.cpp
+++ b/clang/test/CodeGenCXX/pragma-loop.cpp
@@ -278,7 +278,7 @@ void for_test_interleave_vectorize_disable_unroll(int *List, int Length) {
 
 // CHECK-DAG: ![[LOOP_4]] = distinct !{![[LOOP_4]], ![[WIDTH_2]], ![[FIXED_VEC]], ![[INTERLEAVE_2]], ![[VECTORIZE_ENABLE]]}
 
-// CHECK-DAG: ![[LOOP_5]] = distinct !{![[LOOP_5]], ![[MP]], ![[UNROLL_DISABLE]], ![[DISTRIBUTE_DISABLE]], ![[WIDTH_1]]}
+// CHECK-DAG: ![[LOOP_5]] = distinct !{![[LOOP_5]], ![[MP]], ![[UNROLL_DISABLE]], ![[DISTRIBUTE_DISABLE]], ![[WIDTH_1]], ![[FIXED_VEC]]}
 
 // CHECK-DAG: ![[LOOP_6]] = distinct !{![[LOOP_6]], ![[MP]], ![[WIDTH_2]], ![[FIXED_VEC]], ![[INTERLEAVE_2]], ![[VECTORIZE_ENABLE]], ![[FOLLOWUP_VECTOR_3]]}
 
@@ -309,5 +309,5 @@ void for_test_interleave_vectorize_disable_unroll(int *List, int Length) {
 // CHECK-DAG: ![[LOOP_19]] = distinct !{![[LOOP_19]], ![[MP]], ![[UNROLL_DISABLE]], ![[DISTRIBUTE_DISABLE]], ![[WIDTH_1]], ![[SCALABLE_VEC]], ![[INTERLEAVE_4]], ![[VECTORIZE_ENABLE]]}
 // CHECK-DAG: ![[LOOP_20]] = distinct !{![[LOOP_20]], ![[MP]], ![[UNROLL_DISABLE]], ![[DISTRIBUTE_DISABLE]], ![[WIDTH_1]], ![[FIXED_VEC]], ![[INTERLEAVE_4]]}
 // CHECK-DAG: ![[LOOP_21]] = distinct !{![[LOOP_21]], ![[MP]], ![[UNROLL_DISABLE]], ![[DISTRIBUTE_DISABLE]], ![[WIDTH_1]], ![[FIXED_VEC]], ![[INTERLEAVE_4]]}
-// CHECK-DAG: ![[LOOP_22]] = distinct !{![[LOOP_22]], ![[MP]], ![[WIDTH_1]], ![[ISVECTORIZED]], ![[UNROLL_8]]}
-// CHECK-DAG: ![[LOOP_23]] = distinct !{![[LOOP_23]], ![[MP]], ![[WIDTH_1]], ![[INTERLEAVE_4]], ![[ISVECTORIZED]], ![[UNROLL_8]]}
+// CHECK-DAG: ![[LOOP_22]] = distinct !{![[LOOP_22]], ![[MP]], ![[WIDTH_1]], ![[FIXED_VEC]], ![[ISVECTORIZED]], ![[UNROLL_8]]}
+// CHECK-DAG: ![[LOOP_23]] = distinct !{![[LOOP_23]], ![[MP]], ![[WIDTH_1]], ![[FIXED_VEC]], ![[INTERLEAVE_4]], ![[ISVECTORIZED]], ![[UNROLL_8]]}