[Bug target/49263] SH Target: underutilized "TST #imm, R0" instruction

["klepikov.alex+bugs at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49263

--- Comment #39 from Alexander Klepikov <klepikov.alex+bugs at gmail dot com> ---
> The tst insn is mainly formed by the combine pass, which relies on certain
> insn patterns and combinations thereof.  See also sh.md, around line 530.

I'm sorry, but .md lang is too complicated for me.

> You can look at the debug output with the -fdump-rtl-all option to see
> what's happening in the RTL passes.

It looks like with optimization enabled it converts bitwise AND to right shift
and then optimizes again. But SH4 has 'shad' and 'shad' can be optimized to
'tst'. And SH2E has libcall instead of dynamic shift and libcll cannot be
converted. It seems that very first optimization spoils things.

But when we have numerous 'shar' instructions, optimization joins the game
again and converts them to 'tst'.

> What your patch is doing is to make it not emit the ashrsi3_n insn for
> constant shifts altogether?  I guess it will make code that actually needs
> those real shifts larger, as it will always emit the whole shift stitching
> sequence.  That might be a good thing or not.

You are absolutely right, the code will be larger when we do right shifts. But
there's situations when you can't use library. For exmple, SH7055 engine
control unit can barely contain the program. The library just won't fit.