Re: [RFC PATCH v3 30/34] Hexagon (target/hexagon) TCG for instructions with multiple definitions

[Richard Henderson <richard.henderson@linaro.org>]

On 8/30/20 2:30 PM, Taylor Simpson wrote:
> 
> 
>> -----Original Message-----
>> From: Richard Henderson <richard.henderson@linaro.org>
>> Sent: Sunday, August 30, 2020 3:13 PM
>> To: Taylor Simpson <tsimpson@quicinc.com>; qemu-devel@nongnu.org
>> Cc: philmd@redhat.com; laurent@vivier.eu; riku.voipio@iki.fi;
>> aleksandar.m.mail@gmail.com; ale@rev.ng
>> Subject: Re: [RFC PATCH v3 30/34] Hexagon (target/hexagon) TCG for
>> instructions with multiple definitions
>>
>> On 8/30/20 12:48 PM, Taylor Simpson wrote:
>>> I'll add the following comment to indicate what's going on
>>>
>>> /*
>>>   * Each of the generated helpers is wrapped with #ifndef
>> fGEN_TCG_<tag>.
>>>   * For example
>>>    *     #ifndef fGEN_TCG_A2_add
>>>    *     DEF_HELPER_3(A2_add, s32, env, s32, s32)
>>>    *     #endif
>>>   * We include gen_tcg.h here to provide definitions of fGEN_TCG for any
>> instructions that
>>>   * are overridden.
>>>   *
>>>   * This prevents unused helpers from taking up space in the executable.
>>>   */
>>
>> Ah, hum.  Well.
>>
>> How about we figure out a way to communicate to the generator scripts
>> which
>> functions have been implemented "directly", and don't need to be
>> generated at all?
>>
>> I don't see why we have to wait until the c preprocessor to remove this
>> unused
>> code, and the less we have of it, the better.
>>
> 
> A few reasons
> - Makes it easy to override instruction helpers.  All one has to do is #define fGEN_TCG_<tag>.

If the generator can examine, say, genptr_override.c.inc, then you don't even
have to add a #define.  Just add the code.

Perhaps something like

DEFINE_FGEN(tag)
{
    // some code
}

where DEFINE_FGEN expands to the appropriate function declaration.  The
generator then need only grep '^DEFINE_FGEN' and extract the list of overridden
tags.


> - When debugging the override, sometimes you want to quickly revert back to the helper.  Or if you've written a bunch of overrides in one shot and then find that a test case is failing, you can binary search which one to turn off and get the test to pass.  This is the one with the bug to fix.

No difference there, just binary searching on different text.

> - Reduces time for an incremental build.  When we add or delete an override, we don't have to re-run the generator.

About this I care not at all.  I can't imagine that more than fractions of a
second are at stake.


r~