https://github.com/statham-arm updated
https://github.com/llvm/llvm-project/pull/151354
>From 7cc5fed6d24f78114607309a5d1d0aba3a1553e4 Mon Sep 17 00:00:00 2001
From: Simon Tatham <simon.tat...@arm.com>
Date: Wed, 30 Jul 2025 13:46:30 +0100
Subject: [PATCH 1/3] [clang][ARM] Fix build failure in <arm_acle.h> for __swp
In commit d5985905ae8e5b2 I introduced a Sema check that prohibits
`__builtin_arm_ldrex` and `__builtin_arm_strex` for data sizes not
supported by the target architecture version. However, `arm_acle.h`
unconditionally uses those builtins with a 32-bit data size. So now
including that header will cause a build failure on Armv6-M, or
historic architectures like Armv5.
To fix it, `arm_acle.h` now queries the compiler-defined
`__ARM_FEATURE_LDREX` macro (also fixed recently in commit
34f59d79209268e so that it matches the target architecture). If 32-bit
LDREX isn't available it will fall back to the older SWP instruction,
or failing that (on Armv6-M), a libcall.
While I was modifying the header anyway, I also renamed the local
variable `v` inside `__swp` so that it starts with `__`, avoiding any
risk of user code having #defined `v`.
---
clang/lib/Headers/arm_acle.h | 22 +++++++++++++++++++---
clang/test/CodeGen/arm_acle_swp.c | 19 +++++++++++++++++++
2 files changed, 38 insertions(+), 3 deletions(-)
create mode 100644 clang/test/CodeGen/arm_acle_swp.c
diff --git a/clang/lib/Headers/arm_acle.h b/clang/lib/Headers/arm_acle.h
index 5cfa3d023a7d5..a74144baadaba 100644
--- a/clang/lib/Headers/arm_acle.h
+++ b/clang/lib/Headers/arm_acle.h
@@ -55,11 +55,27 @@ __chkfeat(uint64_t __features) {
/* 7.5 Swap */
static __inline__ uint32_t __attribute__((__always_inline__, __nodebug__))
__swp(uint32_t __x, volatile uint32_t *__p) {
- uint32_t v;
+ uint32_t __v;
+#if __ARM_FEATURE_LDREX & 4
do
- v = __builtin_arm_ldrex(__p);
+ __v = __builtin_arm_ldrex(__p);
while (__builtin_arm_strex(__x, __p));
- return v;
+#elif !__ARM_ARCH_6M__
+ /* Fall back to the deprecated SWP instruction, on historic architecture
+ * versions without load/store exclusive instructions on 32-bit data. ACLE is
+ * clear that we mustn't use SWP in any _other_ situation, but permits us to
+ * use it if there's no other option. */
+ __asm__("swp %0, %1, [%2]" : "=r"(__v) : "r"(__x), "r"(__p) : "memory");
+#else
+ /* Armv6-M doesn't have either of LDREX or SWP. ACLE suggests this
+ * implementation, which Clang lowers to the 'cmpxchg' operation in LLVM IR.
+ * On Armv6-M, LLVM turns that into a libcall to __atomic_compare_exchange_4,
+ * so the runtime will need to implement that. */
+ do
+ __v = *__p;
+ while (__sync_bool_compare_and_swap(__p, __v, __x));
+#endif
+ return __v;
}
/* 7.6 Memory prefetch intrinsics */
diff --git a/clang/test/CodeGen/arm_acle_swp.c
b/clang/test/CodeGen/arm_acle_swp.c
new file mode 100644
index 0000000000000..3a99737163afe
--- /dev/null
+++ b/clang/test/CodeGen/arm_acle_swp.c
@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 -ffreestanding -triple thumbv7m-none-eabi -O0
-disable-O0-optnone -emit-llvm -o - %s | opt -S -passes=mem2reg | FileCheck %s
-check-prefix=LDREX
+// RUN: %clang_cc1 -ffreestanding -triple armv7a-none-eabi -O0
-disable-O0-optnone -emit-llvm -o - %s | opt -S -passes=mem2reg | FileCheck %s
-check-prefix=LDREX
+// RUN: %clang_cc1 -ffreestanding -triple armv6-none-eabi -O0
-disable-O0-optnone -emit-llvm -o - %s | opt -S -passes=mem2reg | FileCheck %s
-check-prefix=LDREX
+// RUN: %clang_cc1 -ffreestanding -triple thumbv6m-none-eabi -O0
-disable-O0-optnone -emit-llvm -o - %s | opt -S -passes=mem2reg | FileCheck %s
-check-prefix=SYNC
+// RUN: %clang_cc1 -ffreestanding -triple armv5-none-eabi -O0
-disable-O0-optnone -emit-llvm -o - %s | opt -S -passes=mem2reg | FileCheck %s
-check-prefix=SWP
+
+// REQUIRES: arm-registered-target
+
+#include <arm_acle.h>
+
+// LDREX: call i32 @llvm.arm.ldrex.p0(ptr elementtype(i32) {{.*}})
+// LDREX: call i32 @llvm.arm.strex.p0(i32 {{.*}}, ptr elementtype(i32) {{.*}})
+
+// SWP: call i32 asm "swp $0, $1, [$2]", "=r,r,r,~{memory}"(i32 {{.*}}, ptr
{{.*}})
+
+// SYNC: cmpxchg ptr {{.*}}, i32 {{.*}}, i32 {{.*}} seq_cst seq_cst, align 4
+uint32_t test_swp(uint32_t x, volatile void *p) {
+ return __swp(x, p);
+}
>From f95f6076c91bbdee5220d7d41f2b549d206bde47 Mon Sep 17 00:00:00 2001
From: Simon Tatham <simon.tat...@arm.com>
Date: Thu, 31 Jul 2025 09:58:01 +0100
Subject: [PATCH 2/3] Use __atomic_exchange_n to improve the v6-M case
---
clang/lib/Headers/arm_acle.h | 11 ++++-------
clang/test/CodeGen/arm_acle_swp.c | 10 +++++-----
2 files changed, 9 insertions(+), 12 deletions(-)
diff --git a/clang/lib/Headers/arm_acle.h b/clang/lib/Headers/arm_acle.h
index a74144baadaba..d769e8310847a 100644
--- a/clang/lib/Headers/arm_acle.h
+++ b/clang/lib/Headers/arm_acle.h
@@ -67,13 +67,10 @@ __swp(uint32_t __x, volatile uint32_t *__p) {
* use it if there's no other option. */
__asm__("swp %0, %1, [%2]" : "=r"(__v) : "r"(__x), "r"(__p) : "memory");
#else
- /* Armv6-M doesn't have either of LDREX or SWP. ACLE suggests this
- * implementation, which Clang lowers to the 'cmpxchg' operation in LLVM IR.
- * On Armv6-M, LLVM turns that into a libcall to __atomic_compare_exchange_4,
- * so the runtime will need to implement that. */
- do
- __v = *__p;
- while (__sync_bool_compare_and_swap(__p, __v, __x));
+ /* Armv6-M doesn't have either of LDREX or SWP. LLVM turns the following
+ * builtin into a libcall to __atomic_exchange_4, so the runtime will need to
+ * implement that. */
+ __v = __atomic_exchange_n(__p, __x, __ATOMIC_RELAXED);
#endif
return __v;
}
diff --git a/clang/test/CodeGen/arm_acle_swp.c
b/clang/test/CodeGen/arm_acle_swp.c
index 3a99737163afe..fe87b8c1f7230 100644
--- a/clang/test/CodeGen/arm_acle_swp.c
+++ b/clang/test/CodeGen/arm_acle_swp.c
@@ -1,19 +1,19 @@
// RUN: %clang_cc1 -ffreestanding -triple thumbv7m-none-eabi -O0
-disable-O0-optnone -emit-llvm -o - %s | opt -S -passes=mem2reg | FileCheck %s
-check-prefix=LDREX
// RUN: %clang_cc1 -ffreestanding -triple armv7a-none-eabi -O0
-disable-O0-optnone -emit-llvm -o - %s | opt -S -passes=mem2reg | FileCheck %s
-check-prefix=LDREX
// RUN: %clang_cc1 -ffreestanding -triple armv6-none-eabi -O0
-disable-O0-optnone -emit-llvm -o - %s | opt -S -passes=mem2reg | FileCheck %s
-check-prefix=LDREX
-// RUN: %clang_cc1 -ffreestanding -triple thumbv6m-none-eabi -O0
-disable-O0-optnone -emit-llvm -o - %s | opt -S -passes=mem2reg | FileCheck %s
-check-prefix=SYNC
+// RUN: %clang_cc1 -ffreestanding -triple thumbv6m-none-eabi -O0
-disable-O0-optnone -emit-llvm -o - %s | opt -S -passes=mem2reg | FileCheck %s
-check-prefix=ATOMIC
// RUN: %clang_cc1 -ffreestanding -triple armv5-none-eabi -O0
-disable-O0-optnone -emit-llvm -o - %s | opt -S -passes=mem2reg | FileCheck %s
-check-prefix=SWP
// REQUIRES: arm-registered-target
#include <arm_acle.h>
-// LDREX: call i32 @llvm.arm.ldrex.p0(ptr elementtype(i32) {{.*}})
-// LDREX: call i32 @llvm.arm.strex.p0(i32 {{.*}}, ptr elementtype(i32) {{.*}})
+// LDREX: call i32 @llvm.arm.ldrex.p0(ptr elementtype(i32) {{.*}})
+// LDREX: call i32 @llvm.arm.strex.p0(i32 {{.*}}, ptr elementtype(i32) {{.*}})
-// SWP: call i32 asm "swp $0, $1, [$2]", "=r,r,r,~{memory}"(i32 {{.*}}, ptr
{{.*}})
+// SWP: call i32 asm "swp $0, $1, [$2]", "=r,r,r,~{memory}"(i32 {{.*}}, ptr
{{.*}})
-// SYNC: cmpxchg ptr {{.*}}, i32 {{.*}}, i32 {{.*}} seq_cst seq_cst, align 4
+// ATOMIC: atomicrmw volatile xchg ptr {{.*}}, i32 {{.*}} monotonic, align 4
uint32_t test_swp(uint32_t x, volatile void *p) {
return __swp(x, p);
}
>From bfa6a3d73b7c542017d40e3ea458346a8fd2af78 Mon Sep 17 00:00:00 2001
From: Simon Tatham <simon.tat...@arm.com>
Date: Fri, 1 Aug 2025 11:55:26 +0100
Subject: [PATCH 3/3] Use __atomic_exchange_n for modern systems and Linux too
---
clang/lib/Headers/arm_acle.h | 41 ++++++++++++++++++++++++------------
1 file changed, 27 insertions(+), 14 deletions(-)
diff --git a/clang/lib/Headers/arm_acle.h b/clang/lib/Headers/arm_acle.h
index d769e8310847a..fcc2075121b44 100644
--- a/clang/lib/Headers/arm_acle.h
+++ b/clang/lib/Headers/arm_acle.h
@@ -56,21 +56,34 @@ __chkfeat(uint64_t __features) {
static __inline__ uint32_t __attribute__((__always_inline__, __nodebug__))
__swp(uint32_t __x, volatile uint32_t *__p) {
uint32_t __v;
-#if __ARM_FEATURE_LDREX & 4
- do
- __v = __builtin_arm_ldrex(__p);
- while (__builtin_arm_strex(__x, __p));
-#elif !__ARM_ARCH_6M__
- /* Fall back to the deprecated SWP instruction, on historic architecture
- * versions without load/store exclusive instructions on 32-bit data. ACLE is
- * clear that we mustn't use SWP in any _other_ situation, but permits us to
- * use it if there's no other option. */
- __asm__("swp %0, %1, [%2]" : "=r"(__v) : "r"(__x), "r"(__p) : "memory");
-#else
- /* Armv6-M doesn't have either of LDREX or SWP. LLVM turns the following
- * builtin into a libcall to __atomic_exchange_4, so the runtime will need to
- * implement that. */
+#if (__ARM_FEATURE_LDREX & 4) || __ARM_ARCH_6M__ || __linux__
+ /*
+ * Using this clang builtin is sensible in most situations. Where
+ * LDREX and STREX are available, it will compile to a loop using
+ * them. Otherwise it will compile to a libcall, requiring the
+ * runtime to provide that library function.
+ *
+ * That's unavoidable on Armv6-M, which has no atomic instructions
+ * at all (not even SWP), so in that situation the user will just
+ * have to provide an implementation of __atomic_exchange_4 (perhaps
+ * it would temporarily disable interrupts, and then do a separate
+ * load and store).
+ *
+ * We also use the libcall strategy on pre-Armv7 Linux targets, on
+ * the theory that Linux's runtime support library _will_ provide a
+ * suitable libcall, and it's better to use that than the SWP
+ * instruction because then when the same binary is run on a later
+ * Linux system the libcall implementation will use LDREX instead.
+ */
__v = __atomic_exchange_n(__p, __x, __ATOMIC_RELAXED);
+#else
+ /*
+ * But for older Arm architectures when the target is not Linux, we
+ * fall back to using the SWP instruction via inline assembler. ACLE
+ * is clear that we're allowed to do this, but shouldn't do it if we
+ * have a better alternative.
+ */
+ __asm__("swp %0, %1, [%2]" : "=r"(__v) : "r"(__x), "r"(__p) : "memory");
#endif
return __v;
}
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