Hi everyone,
I've been experimenting compiling my project with Clang and have run into some issues with the supervisor calls and their wrapper in nrf_svc.h.
Without optimizations, the calls seem to work fine. See the following asm generated by Clang at -O0 that loads r0 and r1 with the parameters for the SVC:
/* uint32_t rc = sd_softdevice_enable(NRF_CLOCK_LFCLKSRC_XTAL_75_PPM, assertion_handler); */
ldr r1, 0x000188A8 // second parameter <= &assertion_handler()
movs r0, #5 // first parameter <= LFCLK configuration
bl 0x000188B8 <sd_softdevice_enable>
ldr r1, [sp, #16] // Put address of 'rc' into r1
str r0, [r1] // Store return value from SVC into 'rc'
/* ASSERT(rc == NRF_SUCCESS); */
cmp r0, #0 // Check 'rc == NRF_SUCCESS'
beq 0x00018810 // All good, don't assert
b 0x00018802 // Assert, rc != NRF_SUCCESS
<sd_softdevice_enable>
add r2, sp, #8
mov r3, r1
mov r4, r0
str r0, [r2, #4]
str r1, [r2]
svc #16
bx lr
There are some unnecessary moves and stores in <sd_softdevice_enable> (since everything is already in a register), but it appears to work.
To contrast, here's equivalent asm from GCC at -O2 (much more concise):
/* uint32_t rc = sd_softdevice_enable(NRF_CLOCK_LFCLKSRC_XTAL_75_PPM, assertion_handler); */
movs r0, #5 // first parameter <= LFCLK configuration
ldr r1, 0x00018938 // second parameter <= &assertion_handler()
bl 0x000187BC <sd_softdevice_enable>
/* ASSERT(rc == NRF_SUCCESS); */
cmp r0, #0 // Check 'rc == NRF_SUCCESS'
bne 0x00018928 // Assert if !=
<sd_softdevice_enable>
svc #16
bx lr
And here's Clang again at -O2. Everything breaks down.
/* uint32_t rc = sd_softdevice_enable( NRF_CLOCK_LFCLKSRC_XTAL_75_PPM, assertion_handler); */
bl 0x0001889C <sd_softdevice_enable> // Where are the parameter loads into r0 and r1?
// Oh no! It unconditionally asserts!
/* ASSERT(rc == NRF_SUCCESS); */
ldr r4, 0x0001888C
ldr r5, 0x00018890
movs r2, #0x72
mov r0, r4
mov r1, r5
bl 0x000254F4 <uart_fatalOut>
bkpt #0 // R.I.P.
<sd_softdevice_enable> // Same as the previous two
svc #16
bx lr
Summarized, here's what I see at Clang -O2:
- No loading of r0, r1 of the input parameters to the SVC
- No capturing of the return value from sd_softdevice_enable
It's as if Clang isn't following the calling convention.
Here's my hypothesis of why. The following is from nrf_svc.h (just the relevant part):
#define SVCALL(number, return_type, signature) \
__attribute__((naked)) static return_type signature \
{ \
__asm( \
"svc %0\n" \
"bx r14" : : "I" (number) : "r0" \
); \
}
Nordic uses the preprocessor macro SVCALL to define all the sd_* functions implemented as supervisor calls. Here's the definition of sd_softdevice_enable after preprocessing and some improved formatting:
__attribute__((naked)) static uint32_t sd_softdevice_enable(
nrf_clock_lfclksrc_t clock_source,
softdevice_assertion_handler_t assertion_handler)
{
__asm(
"svc %0 \t\n
bx r14 \t\n"
: : "I" (SD_SOFTDEVICE_ENABLE) : "r0"
);
}
Maybe I'm wrong, but I understand that using extended inline assembly is undefined in a naked function. Only basic inline asm, without output:input:clobber lists is allowed. GCC's docs say so here.
Only basic asm statements can safely be included in naked functions (see Basic Asm). While using extended asm or a mixture of basic asm and C code may appear to work, they cannot be depended upon to work reliably and are not supported.
It seems to me that GCC does the "right" thing, and Clang doesn't, however relying on GCC to always do the right thing isn't dependable.
I'm not very experienced at this low-level, but I think that using standard C functions (dropping the static and the naked attribute) might be the solution.
Of course, I could be misinterpreting the problem. I'm interested in hearing any comments to that effect.
Thanks!
