Re: [PATCH v3] EXPR: Emit an truncate if 31+ bits polluted for SImode

[Richard Sandiford <richard.sandiford@arm.com>]

YunQiang Su <syq@gcc.gnu.org> writes:
> On TRULY_NOOP_TRUNCATION_MODES_P (DImode, SImode)) == true platforms,
> if 31 or above bits is polluted by an bitops, we will need an
> truncate. Let's emit one, and mark let's use the same hardreg
> as in and out, the RTL may like:
>
> (insn 21 20 24 2 (set (subreg/s/u:SI (reg/v:DI 200 [ val ]) 0)
>         (truncate:SI (reg/v:DI 200 [ val ]))) "../xx.c":7:29 -1
>      (nil))
>
> We use /s/u flags to mark it as really needed, as in
> combine_simplify_rtx, this insn may be considered as truncated,
> so let's skip this combination.
>
> gcc/ChangeLog:
>         PR: 104914.
>         * combine.cc (try_combine): Skip combine with truncate if
> 	dest is subreg and has /u/s flags on platforms
> 	TRULY_NOOP_TRUNCATION_MODES_P (DImode, SImode)) == true.
> 	* expr.cc (expand_assignment): Emit a truncate insn, if
> 	31+ bits is polluted for SImode.
>
> gcc/testsuite/ChangeLog:
> 	PR: 104914.
> 	* gcc.target/mips/pr104914.c: New testcase.

Sorry for not looking at this earlier.  I've got a bit lost in the
various threads, so apologies if this has been discussed already
but I think the fix is:

diff --git a/gcc/expr.cc b/gcc/expr.cc
index 4f42c0ff487..9847eba19fe 100644
--- a/gcc/expr.cc
+++ b/gcc/expr.cc
@@ -6275,9 +6275,8 @@ expand_assignment (tree to, tree from, bool nontemporal)
 	      else
 		{
 		  rtx to_rtx1
-		    = lowpart_subreg (subreg_unpromoted_mode (to_rtx),
-				      SUBREG_REG (to_rtx),
-				      subreg_promoted_mode (to_rtx));
+		    = convert_to_mode (subreg_unpromoted_mode (to_rtx),
+				       SUBREG_REG (to_rtx), false);
 		  result = store_field (to_rtx1, bitsize, bitpos,
 					bitregion_start, bitregion_end,
 					mode1, from, get_alias_set (to),

(completely untested apart from the test case).  That should still
produce a subreg on most targets, but generates the required trunc
on MIPS.

Thanks,
Richard