[PATCH] Also preserve SUBREG_PROMOTED_VAR_P in expr.c's convert_move.

[Roger Sayle]

This patch catches another place in the middle-end where it's possible
to preserve the SUBREG_PROMOTED_VAR_P annotation on a subreg to the
benefit of later RTL optimizations.  This adds the same logic to
expr.c's convert_move as recently added to convert_modes.

On nvptx-none, the simple test program:

short foo (char c) { return c; }

currently generates three instructions:

mov.u32 %r23, %ar0;
cvt.u16.u32     %r24, %r23;
cvt.s32.s16     %value, %r24;

with this patch, we now generate just one:

mov.u32 %value, %ar0;


This patch should look familiar, it's almost identical to the recent patch
https://gcc.gnu.org/pipermail/gcc-patches/2021-August/578331.html but with
the fix https://gcc.gnu.org/pipermail/gcc-patches/2021-August/578519.html
[Apologies again for this breakage on affected (non-x86_64) targets; I hope
having that fixed within a few hours (before many folks even noticed a
problem) minimized the inconvenience].

This patch has been tested on x86_64-pc-linux-gnu with "make bootstrap"
and "make -k check" with no new failures, and on a cross-compiler to
nvptx-none, with no new failures in its testsuite.  OK for mainline?


2021-09-11  Roger Sayle  <ro...@nextmovesoftware.com>

gcc/ChangeLog
        * expr.c (convert_move): Preserve SUBREG_PROMOTED_VAR_P when
        creating a (wider) partial subreg from a SUBREG_PROMOTED_VAR_P
        subreg.

Roger
--


diff --git a/gcc/expr.c b/gcc/expr.c
index 17f2c2f..e0bcbcc 100644
--- a/gcc/expr.c
+++ b/gcc/expr.c
@@ -236,8 +236,27 @@ convert_move (rtx to, rtx from, int unsignedp)
          >= GET_MODE_PRECISION (to_int_mode))
       && SUBREG_CHECK_PROMOTED_SIGN (from, unsignedp))
     {
+      scalar_int_mode int_orig_mode;
+      scalar_int_mode int_inner_mode;
+      machine_mode orig_mode = GET_MODE (from);
+
       from = gen_lowpart (to_int_mode, SUBREG_REG (from));
       from_mode = to_int_mode;
+
+      /* Preserve SUBREG_PROMOTED_VAR_P if the new mode is wider than
+        the original mode, but narrower than the inner mode.  */
+      if (GET_CODE (from) == SUBREG
+         && is_a <scalar_int_mode> (orig_mode, &int_orig_mode)
+         && GET_MODE_PRECISION (to_int_mode)
+            > GET_MODE_PRECISION (int_orig_mode)
+         && is_a <scalar_int_mode> (GET_MODE (SUBREG_REG (from)),
+                                    &int_inner_mode)
+         && GET_MODE_PRECISION (int_inner_mode)
+            > GET_MODE_PRECISION (to_int_mode))
+       {
+         SUBREG_PROMOTED_VAR_P (from) = 1;
+         SUBREG_PROMOTED_SET (from, unsignedp);
+       }
     }
 
   gcc_assert (GET_CODE (to) != SUBREG || !SUBREG_PROMOTED_VAR_P (to));