Re: [Bug inline-asm/11103] [3.3/3.4 regression] asm-specifier conflicts with asm clobber list

[Hans-Peter Nilsson <hp at bitrange dot com>]

On Mon, 9 Jun 2003, Richard Earnshaw wrote:
> gcc-bugzilla@gcc.gnu.org said:
> > Confirmed with gcc 3.3 and mainline (20030608). I think this belongs
> > in the inline-asm category  though. Richard, would you mind commenting
> > on whether the code in question is legal? Thanks,
>
> The error is perfectly correct, as I see it, and the manual supports this
> view:

Agreed.  (Whew, I don't have to argue for it again. :-)

> Note, I think the whole asm construct here is somewhat bogus, the compiler
> could legitimately produce the following code for it and claim that it has
> met the "specification" requested:

No it can't.  It is written in stone^W^Win the manual that
asm-declared registers that feed into asms in the source aren't
moved around and renamed by GCC to appear in different registers
in the asm.  Modulo GCC bugs, of course, so I suggest add checks
in the asm.  See for example gcc.dg/asmreg-1.c.

> 	stmfd	sp!, {r4, lr}
> 	mov	r3, r1		// myr1 into safe registers (myr1 is now dead).
> 	mov	r4, r2
> 	mov	r1, r0
> 	// Input registers are r1, r3 and r4.  Output regs are r1 (tied to input)
> 	bl  __put_user_1	// User's asm instruction
> 	mov	r0, r1		// Assign r1 result (from asm) to res.
> 	ldmfd	sp!, {r4, pc}
>
> The key point here is that nothing in the ASM can say which specific
> register an input argument must appear in or where the result will be

By feeding specific asm-declared variables, it does.

> returned -- that might be a weakness in the way our ASM is specified, but
> that is life.

It would be, if it wasn't documented to work (Explicit Reg
Vars):

These local variables are sometimes convenient for use with the
extended @code{asm} feature (@pxref{Extended Asm}), if you want
to write one output of the assembler instruction directly into a
particular register.  (This will work provided the register you
specify fits the constraints specified for that operand in the
@code{asm}.)

Maybe it can bw said in stronger words, but it *is* documented.

brgds, H-P
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