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

[Taylor Simpson]

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