Hi Sara,
----- Original Message -----
From: "Sara Golemon"
Sent: Wednesday, June 10, 2015
On Tue, Jun 9, 2015 at 6:05 AM, Matt Wilmas <php_li...@realplain.com>
wrote:
Hah, looks like this just changed last week after barely 3 weeks. :-P I
didn't verify, just noticed the code change:
http://git.php.net/?p=php-src.git;a=commitdiff;h=c09698753e7d1d95299dca54c8ca888c885fd45b
Dmitry, what's the reasoning behind this diff in the first place?
Doesn't the compiler fold (<const-string> . <const-string>) already
anyhow? How would we wind up with CONCAT_CONST_CONST at runtime?
Now CONCAT is consistent, but not others. Which places need changing
anyway? Just binary ops in VM?
A couple/few extra instructions are unavoidable, I guess on any
architecture, for the intermediate save of op1, but can anyone show that
it
makes ANY measurable difference (other than instruction count)?
I'm 90% certain that fixing the ordering (by saving to a temp var)
doesn't *actually* result in extra instructions. GCC's optimizer is
pretty clever. It'll notice you're only writing to the temp var once,
and reading from it once (immediately), so it'll end up organizing the
instructions in a way which satisfies the explicit order, but as an
inline call.
I don't know... Yeah, if op1 is coming from a "plain" variable, either
something simple (like CONST?) or after function inlining. e.g. Like
"directly" accessible, then it should be able to be done the same. (I don't
know a lot about any of this though.)
I was thinking of when some sort of function call is involved (as far as
unavoidable extra instructions). If op1 is fetched after op2, it's
optimized for passing to concat_function(), etc. Doing stuff in reverse
order will need fewer instructions than "human order," at least when
function calls are needed for "stuff."
Hmm, maybe that's only for x86 with push'ed args, where each thing can keep
push'ing whatever it needs (eax) for the final (outer) function... x86_64
is I guess still going to need to store stuff "out of the way" for the next
(inner) call, before moving it back to the necessary registers for the final
function! If that's true, maybe x86_64 doesn't need more instructions.
I think I've confused myself more trying to picture what's happening. :-O
-Sara
- Matt
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