Implementing SHLD/SHRD instructions in C - c

Question

I'm trying to efficiently implement SHLD and SHRD instructions of x86 without using inline assembly.
uint32_t shld_UB_on_0(uint32_t a, uint32_t b, uint32_t c) {
return a << c | b >> 32 - c;
}
seems to work, but invokes undefined behaviour when c == 0 because the second shift's operand becomes 32. The actual SHLD instruction with third operand being 0 is well defined to do nothing. (https://www.felixcloutier.com/x86/shld)
uint32_t shld_broken_on_0(uint32_t a, uint32_t b, uint32_t c) {
return a << c | b >> (-c & 31);
}
doesn't invoke undefined behaviour, but when c == 0 the result is a | b instead of a.
uint32_t shld_safe(uint32_t a, uint32_t b, uint32_t c) {
if (c == 0) return a;
return a << c | b >> 32 - c;
}
does what's intended, but gcc now puts a je. clang on the other hand is smart enough to translate it to a single shld instruction.
Is there any way to implement it correctly and efficiently without inline assembly?
And why is gcc trying so much not to put shld? The shld_safe attempt is translated by gcc 11.2 -O3 as (Godbolt):
shld_safe:
mov eax, edi
test edx, edx
je .L1
mov ecx, 32
sub ecx, edx
shr esi, cl
mov ecx, edx
sal eax, cl
or eax, esi
.L1:
ret
while clang does,
shld_safe:
mov ecx, edx
mov eax, edi
shld eax, esi, cl
ret

Answer 1

As far as I have tested with gcc 9.3 (x86-64), it translates the following code to shldq and shrdq.
uint64_t shldq_x64(uint64_t low, uint64_t high, uint64_t count) {
return (uint64_t)(((((unsigned __int128)high << 64) | (unsigned __int128)low) << (count & 63)) >> 64);
}
uint64_t shrdq_x64(uint64_t low, uint64_t high, uint64_t count) {
return (uint64_t)((((unsigned __int128)high << 64) | (unsigned __int128)low) >> (count & 63));
}
Also, gcc -m32 -O3 translates the following code to shld and shrd. (I have not tested with gcc (i386), though.)
uint32_t shld_x86(uint32_t low, uint32_t high, uint32_t count) {
return (uint32_t)(((((uint64_t)high << 32) | (uint64_t)low) << (count & 31)) >> 32);
}
uint32_t shrd_x86(uint32_t low, uint32_t high, uint32_t count) {
return (uint32_t)((((uint64_t)high << 32) | (uint64_t)low) >> (count & 31));
}
(I have just read the gcc code and written the above functions, i.e. I'm not sure they are your expected ones.)