inline asm semantics: output constraint width smaller than input

From: Török Edwin
Date: Fri Jan 23 2009 - 12:57:36 EST


I am trying to build the kernel with LLVM 2.5 prerelease (using
llvm-gcc-4.2 frontend), however I am running into some inline asm
semantics issues, and after some discussion on LLVM bugzilla I would
like to know if you would be accepting patches for this:

The problem is when "a" output constraint is used with a variable of
smaller width than the
"0" input constraint.

Here are 2 examples:

int __ret_pu; unsigned long __pu_val;
return ({asm volatile("call __put_user_" "8" : "=a" (__ret_pu) :"0"
(__pu_val), "c"(addr) : "ebx"); __ret_pu;});

unsigned char return_code; /* %al */
unsigned long address; /* %ebx */
unsigned long length; /* %ecx */
unsigned long entry; /* %edx */
unsigned long flags;
__asm__("lcall *(%%edi); cld"
: "=a" (return_code),
"=b" (address),
"=c" (length),
"=d" (entry)
: "0" (service),
"1" (0),
"D" (&bios32_indirect));

There are 2 cases:
1. output is wider than input
2. output is narrower than input

Case 2 seems to occur lots of times on 64-bit (due to sizeof(int) != sizeof(unsigned long)),
and a few times on 32-bit as well.

Would you accept patches that increase the portability of the inline asm statements?
(essentially by adding casts for case 1, and introducing a temporary of correct width for case 2).

Please see:

Best regards,

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