[LLVMbugs] [Bug 9517] New: Input constraints for inline assembler are not used for CSE

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=9517

           Summary: Input constraints for inline assembler are not used
                    for CSE
           Product: new-bugs
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: joerg at NetBSD.org
                CC: llvmbugs at cs.uiuc.edu


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.

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