emit-rtl.c (operand_subword): Tighten checks for when it is safe to safe to extract a subword out of a REG.

        * emit-rtl.c (operand_subword): Tighten checks for when it is safe
        to safe to extract a subword out of a REG.

From-SVN: r27564

1 files changed, 26 insertions, 3 deletions

diff --git a/gcc/emit-rtl.c b/gcc/emit-rtl.c
index 2aa51ac..f1caea7 100644
--- a/gcc/emit-rtl.c
+++ b/gcc/emit-rtl.c
@@ -1196,10 +1196,33 @@ operand_subword (op, i, validate_address, mode)
   /* If OP is a REG or SUBREG, we can handle it very simply.  */
   if (GET_CODE (op) == REG)
     {
-      /* If the register is not valid for MODE, return 0.  If we don't
-	 do this, there is no way to fix up the resulting REG later.  */
+      /* ??? There is a potential problem with this code.  It does not
+	 properly handle extractions of a subword from a hard register
+	 that is larger than word_mode.  Presumably the check for
+	 HARD_REGNO_MODE_OK catches these most of these cases.  */
+
+      /* If OP is a hard register, but OP + I is not a hard register,
+	 then extracting a subword is impossible.
+
+	 For example, consider if OP is the last hard register and it is
+	 larger than word_mode.  If we wanted word N (for N > 0) because a
+	 part of that hard register was known to contain a useful value,
+	 then OP + I would refer to a pseudo, not the hard register we
+	 actually wanted.  */
+      if (REGNO (op) < FIRST_PSEUDO_REGISTER
+	  && REGNO (op) + i >= FIRST_PSEUDO_REGISTER)
+	return 0;
+
+      /* If the register is not valid for MODE, return 0.  Note we
+	 have to check both OP and OP + I since they may refer to
+	 different parts of the register file.
+
+	 Consider if OP refers to the last 96bit FP register and we want
+	 subword 3 because that subword is known to contain a value we
+	 needed.  */
       if (REGNO (op) < FIRST_PSEUDO_REGISTER
-	  && ! HARD_REGNO_MODE_OK (REGNO (op) + i, word_mode))
+	  && (! HARD_REGNO_MODE_OK (REGNO (op), word_mode)
+	      || ! HARD_REGNO_MODE_OK (REGNO (op) + i, word_mode)))
 	return 0;
       else if (REGNO (op) >= FIRST_PSEUDO_REGISTER
 	       || (REG_FUNCTION_VALUE_P (op)