Invalid hard-reg decomposition in lower-subreg

[Richard Sandiford]

lower-subreg.c:can_decompose_p uses the following condition to test
whether a multiword hard register can be decomposed into words:

        return (validate_subreg (word_mode, GET_MODE (x), x, UNITS_PER_WORD)
                && HARD_REGNO_MODE_OK (regno, word_mode));

This doesn't work reliably on MIPS, where a doubleword HI-LO value
cannot be split into independent HI and LO words; the LO word is OK,
but the HI word isn't.  This means that the test works correctly
on big-endian targets, where the invalid HI case comes first,
but not on little-endian ones, where the valid LO one does.

This endian difference is the cause of the mips-sde-elf build failure
that Maciej reported earlier in the week.  Tested on that target and
on x86_64-linux-gnu.  OK to install?

Richard


gcc/
        * lower-subreg.c (can_decompose_p): Check every word of a hard
        register.

Index: gcc/lower-subreg.c
===================================================================
--- gcc/lower-subreg.c  2011-12-14 20:44:42.000000000 +0000
+++ gcc/lower-subreg.c  2011-12-14 21:04:09.000000000 +0000
@@ -634,8 +634,15 @@ can_decompose_p (rtx x)
       unsigned int regno = REGNO (x);
 
       if (HARD_REGISTER_NUM_P (regno))
-       return (validate_subreg (word_mode, GET_MODE (x), x, UNITS_PER_WORD)
-               && HARD_REGNO_MODE_OK (regno, word_mode));
+       {
+         unsigned int byte, num_bytes;
+
+         num_bytes = GET_MODE_SIZE (GET_MODE (x));
+         for (byte = 0; byte < num_bytes; byte += UNITS_PER_WORD)
+           if (simplify_subreg_regno (regno, GET_MODE (x), byte, word_mode) < 
0)
+             return false;
+         return true;
+       }
       else
        return !bitmap_bit_p (subreg_context, regno);
     }