r246991 - When building the alloca for a local variable, set its name

[Chandler Carruth via cfe-commits]

None of my performance concerns were relevant to this change FWIW.

I think the reason that this got "fixed" was because people had a tendancy
to *rely* on the name downstream when we made it always present. =/
Personally, I like having *no* names in a no-asserts build because it
ensures that absolutely no one is relying on them -- we have debug
information for that.

I wonder if the right way to help the debugging scenario (which is very
real and painful) is to expose a flag for this, or to expose a way to take
an IR file with debug info and synthesize nice names for as much as we can
using the debug info?

On Tue, Sep 8, 2015 at 10:47 AM David Blaikie via cfe-commits <
cfe-commits at lists.llvm.org> wrote:

> On Tue, Sep 8, 2015 at 10:26 AM, John McCall <rjmccall at apple.com> wrote:
>
>> On Sep 8, 2015, at 8:25 AM, David Blaikie <dblaikie at gmail.com> wrote:
>> On Tue, Sep 8, 2015 at 2:18 AM, John McCall via cfe-commits <
>> cfe-commits at lists.llvm.org> wrote:
>>
>>> Author: rjmccall
>>> Date: Tue Sep  8 04:18:30 2015
>>> New Revision: 246991
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=246991&view=rev
>>> Log:
>>> When building the alloca for a local variable, set its name
>>> separately from building the instruction so that it's
>>> preserved even in -Asserts builds.
>>>
>>
>> Why do we want to preserve this name in particular in -Asserts builds?
>>
>>
>> It’s a fairly small runtime cost — no twine appending, only present on a
>> tiny fraction of instructions, unlikely to collide within a single function
>> — that makes reading the IR dramatically easier.  That’s still useful to be
>> able to do with a production compiler, both for us and for sophisticated
>> users.
>>
>> IIRC, Daniel did the perf work on IR names way back when; he might have
>> other thoughts.
>>
>
> *nod* I'd be curious to see/hear/better understand the tradeoffs. I know
> I've talked to Chandler (CC'd) about it before when he's expressed a desire
> to want to more fully remove names from non-asserts builds (specifically:
> optimizations still create named values even in -Asserts builds, I think?
> or maybe there was some other - oh, right, as you were saying, the Twine
> building cost, even when the name isn't used some names are complex to
> create, etc)
>
>
>>
>> John.
>>
>
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