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Date: Thu, 9 Jul 2020 13:56:19 -0400 (EDT)
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
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Subject: Re: Failure to build librseq on ppc
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Cc: Boqun Feng <boqun.feng@gmail.com>,
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----- On Jul 9, 2020, at 1:42 PM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:

> ----- On Jul 9, 2020, at 1:37 PM, Segher Boessenkool segher@kernel.crashing.org
> wrote:
> 
>> On Thu, Jul 09, 2020 at 09:43:47AM -0400, Mathieu Desnoyers wrote:
>>> > What protects r17 *after* this asm statement?
>>> 
>>> As discussed in the other leg of the thread (with the code example),
>>> r17 is in the clobber list of all asm statements using this macro, and
>>> is used as a temporary register within each inline asm.
>> 
>> That works fine then, for a testcase.  Using r17 is not a great idea for
>> performance (it increases the active register footprint, and causes more
>> registers to be saved in the prologue of the functions, esp. on older
>> compilers), and it is easier to just let the compiler choose a good
>> register to use.  But maybe you want to see r17 in the generated
>> testcases, as eyecatcher or something, dunno :-)
> 
> Just to make sure I understand your recommendation. So rather than
> hard coding r17 as the temporary registers, we could explicitly
> declare the temporary register as a C variable, pass it as an
> input operand to the inline asm, and then refer to it by operand
> name in the macros using it. This way the compiler would be free
> to perform its own register allocation.
> 
> If that is what you have in mind, then yes, I think it makes a
> lot of sense.

Except that asm goto have this limitation with gcc: those cannot
have any output operand, only inputs, clobbers and target labels.
We cannot modify a temporary register received as input operand. So I don't
see how to get a temporary register allocated by the compiler considering
this limitation.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com