[PATCH v2 1/3] rtl: properly handle subreg (mem) in gen_highpart [PR102125]

[Richard Earnshaw <rearnsha@arm.com>]

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gen_lowpart_general handles forming a SUBREG of a MEM by using
adjust_address to rework and validate a new version of the MEM.
However, gen_highpart does not attempt this and simply returns (SUBREG
(MEM)) if the change is not 'obviously' safe.  Improve on that by
using a similar approach so that gen_lowpart and gen_highpart are
mostly symmetrical in this regard.

gcc/ChangeLog:

	PR target/102125
	* emit-rtl.c (gen_highpart): If simplify_gen_subreg returns
	SUBREG (MEM) for a MEM, use adjust_address to produce a new
	MEM.
---
 gcc/emit-rtl.c | 8 ++++++++
 1 file changed, 8 insertions(+)


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diff --git a/gcc/emit-rtl.c b/gcc/emit-rtl.c
index 77ea8948ee8..bacf6fffa22 100644
--- a/gcc/emit-rtl.c
+++ b/gcc/emit-rtl.c
@@ -1597,6 +1597,14 @@ gen_highpart (machine_mode mode, rtx x)
       result = validize_mem (result);
       gcc_assert (result);
     }
+  /* It may also just put a SUBREG wrapper around a MEM for the same
+     reason.  */
+  else if (GET_CODE (result) == SUBREG && MEM_P (SUBREG_REG (result))
+	   && MEM_P (x))
+    {
+      poly_int64 offset = subreg_highpart_offset (mode, GET_MODE (x));
+      result = adjust_address (x, mode, offset);
+    }
 
   return result;
 }