Hi Richard,
> It seems to be improving TARGET_TRULY_NOOP_TRUNCATION documentation might be
> useful here.
This is an excellent suggestion. How about the following/attached:
2020-07-13 Roger Sayle <ro...@nextmovesoftware.com>
gcc/ChangeLog:
* doc/tm.texi (TARGET_TRULY_NOOP_TRUNCATION): Clarify that targets
that (sometimes) return false, indicating SUBREGs shouldn't be
used, also need to provide a trunc?i?i2 optab that performs this
truncation.
> The only user (after your patch) of this hook is in function.c for the
> function parameter setup btw.
The targetm.truly_noop_truncation in assign_parm_setup_block is the last place
that calls this
hook directly (with sizes), but the majority of uses go via
TRULY_NOOP_TRUNCATION_MODES_P
as defined in machmode.h.
I'll prepare a patch to switch function.c to use TRULY_NOOP_TRUNCATION_MODE_P
so that we
are consistent throughout the compiler. In theory, this hook could then be
changed to take modes
instead of (poly_unit64) sizes, but that clean-up might be tricky without
access to the affected
platforms.
The hard register semantics that you're referring to are related to
TARGET_MODES_TIEABLE_P,
which is documented to have interesting interactions with
TARGET_TRULY_NOOP_TRUNCATION.
Is the above documentation change Ok for mainline?
Thanks,
Roger
--
diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
index 6e7d9dc54a9..41b9e10c856 100644
--- a/gcc/doc/tm.texi
+++ b/gcc/doc/tm.texi
@@ -11122,7 +11122,9 @@ This hook returns true if it is safe to ``convert'' a
value of
@var{inprec} bits to one of @var{outprec} bits (where @var{outprec} is
smaller than @var{inprec}) by merely operating on it as if it had only
@var{outprec} bits. The default returns true unconditionally, which
-is correct for most machines.
+is correct for most machines. When @code{TARGET_TRULY_NOOP_TRUNCATION}
+returns false, the machine description should provide a @code{trunc}
+optab to specify the RTL that performs the required truncation.
If @code{TARGET_MODES_TIEABLE_P} returns false for a pair of modes,
suboptimal code can result if this hook returns true for the corresponding
