gcc miscompiles csum_tcpudp_magic() on ARMv5

Willy Tarreau [off-list ref] writes:

quoted

#include <stdint.h>
#include <stdio.h>

static inline uint32_t asm_add(uint16_t len, uint32_t sum)
{
        __asm__(
        "add    %0, %1, %2 \n"
        : "=&r"(sum)
        : "r" (sum), "r" (len)
        );
        return sum;
}

Hmmm aren't you passing a 16-bit register directly to the ASM for being used
as a 32-bit one ? This seems hasardous to me since nowhere you tell gcc how
you're going to use the register.

There is no such thing as a 16-bit register on ARM.  What this code does
is ask the compiler to take the 16-bit value 'len' and put it in a register
for use in the asm code.  All registers are 32-bit, so there is no ambiguity.

-- 
M?ns Rullg?rd
mans at mansr.com

On Thu, 2013-12-12 at 14:37 +0000, M?ns Rullg?rd wrote:

There is no such thing as a 16-bit register on ARM.  What this code does
is ask the compiler to take the 16-bit value 'len' and put it in a register
for use in the asm code.  All registers are 32-bit, so there is no ambiguity.

which strictly speaking the compiler did ;)

it took the 16 bits value and put it inside the lower 16 bits of the 32
bits register, it just didn't touch the higher ones

-- 
Maxime