Repository navigation
Unnecessary MSVC code #4
Description
The current code uses special casing for MSVC, but this code is likely unneeded. It seems to seek to avoid memcpy calls. But Microsoft's compiler is smart.
Let us consider this example:
#include <cstdint>
#include <cstring>
#define BSWAP_4(x) \
(((x) & 0xFF) << 24) | (((x) & 0xFF00) << 8) | (((x) >> 8) & 0xFF00) | \
(((x) >> 24) & 0xFF)
bool SwapBytes32(char *data, size_t nbytes) {
if (nbytes % sizeof(uint32_t) != 0) return false;
uint32_t temp;
for (size_t i = 0; i < nbytes; i += sizeof(temp)) {
memcpy(&temp, &data[i], sizeof(temp));
temp = BSWAP_4(temp);
memcpy(&data[i], &temp, sizeof(temp));
}
return true;
}
With /O2, MSVC compiles it down to
bool SwapBytes32(char *,unsigned __int64) PROC ; SwapBytes32, COMDAT
test dl, 3
je SHORT $LN5@SwapBytes3
xor al, al
ret 0
$LN5@SwapBytes3:
test rdx, rdx
je SHORT $LN3@SwapBytes3
dec rdx
shr rdx, 2
inc rdx
npad 9
$LL4@SwapBytes3:
mov eax, DWORD PTR [rcx]
lea rcx, QWORD PTR [rcx+4]
bswap eax
mov DWORD PTR [rcx-4], eax
sub rdx, 1
jne SHORT $LL4@SwapBytes3
$LN3@SwapBytes3:
mov al, 1
ret 0I am sure one can do better, but it is just generally fine code.
Activity
lemire commentedon Aug 8, 2024
cc @anonrig
lemire commentedon Aug 8, 2024
(@anonrig not having special case handing for MSVC is likely better long term.)