[PATCH] libiberty: d-demangle: use switch instead of if-else

[Luís Ferreira]

On Sun, 2021-10-03 at 15:55 -0600, Jeff Law wrote:
> 
> 
> On 9/29/2021 7:08 PM, Luís Ferreira wrote:
> > This patch allows the compiler to efficiently generate jump tables
> > instead of
> > using if-else-if.
> > 
> > Signed-off-by: Luís Ferreira <contact@lsferreira.net>
> I'm not sure this is terribly useful.  Compilers have the ability to 
> analyze the underlying code and make sensible decisions for how to 
> implement either form.   So the right metric here is does this make the
> code cleaner/easier to understand.  With just 3 clauses it's hard (for 
> me) to make the case that it is considerably cleaner.
> 
> Jeff
> 
> 

Well, my major point on the patch was performance, eventhough on this
number of clauses it doesn't matter for GCC. I can totally agree with
you.

Although when we talk about a higher number of cases the optimizer
reach a threashold to optimize with jump tables and if-else-if starts
to loose there. Furthermore, if-else-if are hard to optimize, since the
compiler need to check if the condition order is important and with a
high number of cases the compiler may just give up on that check. I'm
not particularly aware of how GCC theoretically optimize it, so take
that with a grain of salt, but, in practice, it sure have a difference.
https://godbolt.org/z/rT9drW117

Even though performance may not be terribly bad (some compilers may not
be really that smart, specially in higher number of cases, as shown
above) I can still consider this change for future additions, even
though debatable, since mangling characters may exapand this logic and
increase the number of cases making if-else-if code a bit longer to
read.

Overall, although, this is by far a minor thing. My intention with this
was to reflect some changes I found relevant while reading this file.

Side note:

This was a previously sent patch. As requested by you, if this is
somehow being considered to be merged, here is the changelog:

ChangeLog:

libiberty/
	* d-demangle.c (dlang_type): Change if-else-if to switch case.

-- 
Sincerely,
Luís Ferreira @ lsferreira.net

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