https://bugs.llvm.org/show_bug.cgi?id=43603
Bug ID: 43603
Summary: C11 atomics always yielding library call with
-mcpu=cortex-m0plus
Product: clang
Version: 9.0
Hardware: Other
OS: other
Status: NEW
Severity: enhancement
Priority: P
Component: C
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected],
[email protected]
Using this minimum example code:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#include <stdatomic.h>
void foo(void);
extern atomic_int_least16_t bar;
void foo(void)
{
atomic_store_explicit(&bar, 0x1337, memory_order_relaxed);
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Compiling with "clang -mcpu=cortex-m3 -mlittle-endian -mthumb -mfloat-abi=soft
-target arm-none-eabi -Weverything -Werror -c -o test.o test.c" works fine.
Compiling with "clang -mcpu=cortex-m0plus -mlittle-endian -mthumb
-mfloat-abi=soft -target arm-none-eabi -Weverything -Werror -c -o test.o
test.c" yields:
> test.c:8:2: error: large atomic operation may incur significant performance
> penalty [-Werror,-Watomic-alignment]
> atomic_store_explicit(&bar, 0x1337, memory_order_relaxed);
> ^
> /usr/lib/clang/9.0.0/include/stdatomic.h:118:31: note: expanded from macro
> 'atomic_store_explicit'
> #define atomic_store_explicit __c11_atomic_store
> ^
> 1 error generated.
However, compiling with "arm-none-eabi-gcc -mcpu=cortex-m0plus -mlittle-endian
-mthumb -mfloat-abi=soft -Wall -Wextra -pedantic -Werror -c -o test.o test.c"
works fine and generates:
> arm-none-eabi-objdump -d test.o
>
> test.o: file format elf32-littlearm
>
>
> Disassembly of section .text:
>
> 00000000 <foo>:
> 0: b580 push {r7, lr}
> 2: b082 sub sp, #8
> 4: af00 add r7, sp, #0
> 6: 4b07 ldr r3, [pc, #28] ; (24 <foo+0x24>)
> 8: 607b str r3, [r7, #4]
> a: 1cbb adds r3, r7, #2
> c: 4a06 ldr r2, [pc, #24] ; (28 <foo+0x28>)
> e: 801a strh r2, [r3, #0]
> 10: 1cbb adds r3, r7, #2
> 12: 2200 movs r2, #0
> 14: 5e9b ldrsh r3, [r3, r2]
> 16: b29a uxth r2, r3
> 18: 687b ldr r3, [r7, #4]
> 1a: 801a strh r2, [r3, #0]
> 1c: 46c0 nop ; (mov r8, r8)
> 1e: 46bd mov sp, r7
> 20: b002 add sp, #8
> 22: bd80 pop {r7, pc}
> 24: 00000000 .word 0x00000000
> 28: 00001337 .word 0x00001337
Therefore, I assume that the atomic store can be implemented without resorting
a a library call to __atomic_store_2().
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