Input constraints for inline assembler are not used for CSE #9889
Description
| Bugzilla Link | 9517 |
| Version | trunk |
| OS | Linux |
| CC | @echristo,@efriedma-quic |
Extended Description
Consider the following program:
static inline void
outb(unsigned short port, unsigned char data)
{
__asm volatile("outb %%al,%w1" : : "a" (data), "d" (port));
}
extern unsigned short base;
void test(void)
{
outb(base + 16, 0x0);
outb(base + 16, 0x1);
outb(base + 16, 0x2);
outb(base + 16, 0x3);
outb(base + 16, 0x4);
outb(base + 16, 0x5);
outb(base + 16, 0x6);
outb(base + 16, 0x7);
outb(base + 16, 0x8);
}
This should be compiled into something like:
movzwl base, %edx
add 0x10, %edx
xorl %eax, %eax
outb %al,%dx
movb $1, %al
outb %al,%dx
movb $2, %al
outb %al,%dx
...
or even better in terms of code size:
movzwl base, %edx
add 0x10, %edx
xorl %eax, %eax
outb %al,%dx
incl %eax
outb %al,%dx
incl %eax
outb %al,%dx
...
But currently it gets compiled to:
movzwl base, %edx
add 0x10, %edx
xorl %eax, %eax
outb %al,%dx
movzwl base, %edx
add 0x10, %edx
movb $1, %al
outb %al,%dx
movzwl base, %edx
add 0x10, %edx
movb $2, %al
outb %al,%dx
...
Input constraints are not clobbered and therefore, they don't have to be reloaded. The reload overhead in this case is significant.