[clang] [Clang][ARM] Fix __ARM_FEATURE_LDREX on Armv8-M (PR #149538)

[Simon Tatham via cfe-commits]

https://github.com/statham-arm created https://github.com/llvm/llvm-project/pull/149538

The Armv8-M architecture doesn't have the LDREXD and STREXD instructions, for exclusive load/store of a 64-bit quantity split across two registers. But the `__ARM_FEATURE_LDREX` macro was set to a value that claims it does, because the case for Armv8 was missing a check for M profile.

The Armv7 case got it right, so I've just made the two cases the same.

>From 9eb51f1ca408747f82ab07adad5bbcc06e323d23 Mon Sep 17 00:00:00 2001
From: Simon Tatham <simon.tatham at arm.com>
Date: Thu, 17 Jul 2025 16:33:33 +0100
Subject: [PATCH] [Clang][ARM] Fix __ARM_FEATURE_LDREX on Armv8-M

The Armv8-M architecture doesn't have the LDREXD and STREXD
instructions, for exclusive load/store of a 64-bit quantity split
across two registers. But the `__ARM_FEATURE_LDREX` macro was set to
a value that claims it does, because the case for Armv8 was missing a
check for M profile.

The Armv7 case got it right, so I've just made the two cases the same.
---
 clang/lib/Basic/Targets/ARM.cpp        |  4 +++-
 clang/test/Preprocessor/arm-acle-6.4.c | 31 ++++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Basic/Targets/ARM.cpp b/clang/lib/Basic/Targets/ARM.cpp
index 7ff8e51f8a7f8..29de34bbc4fe4 100644
--- a/clang/lib/Basic/Targets/ARM.cpp
+++ b/clang/lib/Basic/Targets/ARM.cpp
@@ -623,13 +623,15 @@ bool ARMTargetInfo::handleTargetFeatures(std::vector<std::string> &Features,
       LDREX = LDREX_W;
     break;
   case 7:
+  case 8:
     if (ArchProfile == llvm::ARM::ProfileKind::M)
       LDREX = LDREX_W | LDREX_H | LDREX_B;
     else
       LDREX = LDREX_D | LDREX_W | LDREX_H | LDREX_B;
     break;
-  case 8:
   case 9:
+    assert(ArchProfile != llvm::ARM::ProfileKind::M &&
+           "No Armv9-M architectures defined");
     LDREX = LDREX_D | LDREX_W | LDREX_H | LDREX_B;
   }
 
diff --git a/clang/test/Preprocessor/arm-acle-6.4.c b/clang/test/Preprocessor/arm-acle-6.4.c
index 2c8f4868263a6..48deba74c8ab2 100644
--- a/clang/test/Preprocessor/arm-acle-6.4.c
+++ b/clang/test/Preprocessor/arm-acle-6.4.c
@@ -188,6 +188,37 @@
 // RUN: %clang --target=arm-arm-none-eabi -mcpu=cortex-m33 -x c -E -dM %s -o - | FileCheck %s -check-prefix CHECK-M-DSP
 // RUN: %clang --target=arm-arm-none-eabi -march=armv8m.main+dsp -x c -E -dM %s -o - | FileCheck %s -check-prefix CHECK-M-DSP
 
+// RUN: %clang -target arm-none-linux-eabi -march=armv8m.base -x c -E -dM %s -o - | FileCheck %s -check-prefix CHECK-V8M-BASE
+
+// CHECK-V8M-BASE-NOT: __ARM_ARCH_ISA_ARM
+// CHECK-V8M-BASE-NOT: __ARM_FEATURE_DSP
+// CHECK-V8M-BASE-NOT: __ARM_FEATURE_SIMD32
+// CHECK-V8M-BASE:     __ARM_ARCH 8
+// CHECK-V8M-BASE:     __ARM_ARCH_ISA_THUMB 1
+// CHECK-V8M-BASE:     __ARM_ARCH_PROFILE 'M'
+// CHECK-V8M-BASE:     __ARM_FEATURE_CLZ 1
+// CHECK-V8M-BASE:     __ARM_FEATURE_IDIV 1
+// CHECK-V8M-BASE:     __ARM_FEATURE_LDREX 0x7
+// CHECK-V8M-BASE:     __ARM_FEATURE_QBIT 1
+// CHECK-V8M-BASE:     __ARM_FEATURE_SAT
+// CHECK-V8M-BASE-NOT: __ARM_FEATURE_UNALIGNED
+
+// RUN: %clang -target arm-none-linux-eabi -march=armv8m.main -x c -E -dM %s -o - | FileCheck %s -check-prefix CHECK-V8M-MAIN
+// RUN: %clang -target arm-none-linux-eabi -march=armv8.1m.main -x c -E -dM %s -o - | FileCheck %s -check-prefix CHECK-V8M-MAIN
+
+// CHECK-V8M-MAIN-NOT: __ARM_ARCH_ISA_ARM
+// CHECK-V8M-MAIN-NOT: __ARM_FEATURE_DSP
+// CHECK-V8M-MAIN-NOT: __ARM_FEATURE_SIMD32
+// CHECK-V8M-MAIN:     __ARM_ARCH 8
+// CHECK-V8M-MAIN:     __ARM_ARCH_ISA_THUMB 2
+// CHECK-V8M-MAIN:     __ARM_ARCH_PROFILE 'M'
+// CHECK-V8M-MAIN:     __ARM_FEATURE_CLZ 1
+// CHECK-V8M-MAIN:     __ARM_FEATURE_IDIV 1
+// CHECK-V8M-MAIN:     __ARM_FEATURE_LDREX 0x7
+// CHECK-V8M-MAIN:     __ARM_FEATURE_QBIT 1
+// CHECK-V8M-MAIN:     __ARM_FEATURE_SAT 1
+// CHECK-V8M-MAIN:     __ARM_FEATURE_UNALIGNED 1
+
 // CHECK-M-DSP: __ARM_FEATURE_DSP 1
 // CHECK-M-DSP: __ARM_FEATURE_SIMD32 1