Add subreg_memory_offset helper functions

This patch adds routines for converting a SUBREG_BYTE offset into a
memory address offset.  The two only differ for paradoxical subregs,
where SUBREG_BYTE is always 0 but the memory address offset can be
negative.

2017-09-04  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* rtl.h (subreg_memory_offset): Declare.
	* emit-rtl.c (subreg_memory_offset): New function.
	* expmed.c (store_bit_field_1): Use it.
	* expr.c (undefined_operand_subword_p): Likewise.
	* simplify-rtx.c (simplify_subreg): Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r251644

1 files changed, 27 insertions, 0 deletions

diff --git a/gcc/emit-rtl.c b/gcc/emit-rtl.c
index 3954648..ac6637d 100644
--- a/gcc/emit-rtl.c
+++ b/gcc/emit-rtl.c
@@ -1005,6 +1005,33 @@ byte_lowpart_offset (machine_mode outer_mode,
   else
     return subreg_lowpart_offset (outer_mode, inner_mode);
 }
+
+/* Return the offset of (subreg:OUTER_MODE (mem:INNER_MODE X) OFFSET)
+   from address X.  For paradoxical big-endian subregs this is a
+   negative value, otherwise it's the same as OFFSET.  */
+
+int
+subreg_memory_offset (machine_mode outer_mode, machine_mode inner_mode,
+		      unsigned int offset)
+{
+  if (paradoxical_subreg_p (outer_mode, inner_mode))
+    {
+      gcc_assert (offset == 0);
+      return -subreg_lowpart_offset (inner_mode, outer_mode);
+    }
+  return offset;
+}
+
+/* As above, but return the offset that existing subreg X would have
+   if SUBREG_REG (X) were stored in memory.  The only significant thing
+   about the current SUBREG_REG is its mode.  */
+
+int
+subreg_memory_offset (const_rtx x)
+{
+  return subreg_memory_offset (GET_MODE (x), GET_MODE (SUBREG_REG (x)),
+			       SUBREG_BYTE (x));
+}
 
 /* Generate a REG rtx for a new pseudo register of mode MODE.
    This pseudo is assigned the next sequential register number.  */