rtlanal.c (subreg_lsb_1): New function split out from subreg_lsb.

	* rtlanal.c (subreg_lsb_1): New function split out from subreg_lsb.
	(subreg_lsb): Change to call new subreg_lsb_1 helper function.
	* rtl.h (subreg_lsb_1): Prototype here.
	* simplify-rtx.c (simplify_subreg): Optimize subregs of zero and
	sign extensions.

Co-Authored-By: Paolo Bonzini <bonzini@gnu.org>

From-SVN: r76352

gcc/ChangeLog

+2004-01-22  Roger Sayle  <roger@eyesopen.com>
+	    Paolo Bonzini  <bonzini@gnu.org>
+
+	* rtlanal.c (subreg_lsb_1): New function split out from subreg_lsb.
+	(subreg_lsb): Change to call new subreg_lsb_1 helper function.
+	* rtl.h (subreg_lsb_1): Prototype here.
+	* simplify-rtx.c (simplify_subreg): Optimize subregs of zero and
+	sign extensions.
+
 2004-01-22  Kazu Hirata  <kazu@cs.umass.edu>
 
 	* doc/tm.texi (CASE_VECTOR_PC_RELATIVE): Mention that the

gcc/rtl.h

@@ -1065,6 +1065,8 @@ enum label_kind
 
 /* in rtlanal.c */
 extern unsigned int subreg_lsb (rtx);
+extern unsigned int subreg_lsb_1 (enum machine_mode, enum machine_mode,
+				  unsigned int);
 extern unsigned int subreg_regno_offset	(unsigned int, enum machine_mode,
 					 unsigned int, enum machine_mode);
 extern bool subreg_offset_representable_p (unsigned int, enum machine_mode,

gcc/rtlanal.c

@@ -3187,48 +3187,59 @@ loc_mentioned_in_p (rtx *loc, rtx in)
   return 0;
 }
 
-/* Given a subreg X, return the bit offset where the subreg begins
-   (counting from the least significant bit of the reg).  */
+/* Helper function for subreg_lsb.  Given a subreg's OUTER_MODE, INNER_MODE,
+   and SUBREG_BYTE, return the bit offset where the subreg begins
+   (counting from the least significant bit of the operand).  */
 
 unsigned int
-subreg_lsb (rtx x)
+subreg_lsb_1 (enum machine_mode outer_mode,
+	      enum machine_mode inner_mode,
+	      unsigned int subreg_byte)
 {
-  enum machine_mode inner_mode = GET_MODE (SUBREG_REG (x));
-  enum machine_mode mode = GET_MODE (x);
   unsigned int bitpos;
   unsigned int byte;
   unsigned int word;
 
   /* A paradoxical subreg begins at bit position 0.  */
-  if (GET_MODE_BITSIZE (mode) > GET_MODE_BITSIZE (inner_mode))
+  if (GET_MODE_BITSIZE (outer_mode) > GET_MODE_BITSIZE (inner_mode))
     return 0;
 
   if (WORDS_BIG_ENDIAN != BYTES_BIG_ENDIAN)
     /* If the subreg crosses a word boundary ensure that
        it also begins and ends on a word boundary.  */
-    if ((SUBREG_BYTE (x) % UNITS_PER_WORD
-	 + GET_MODE_SIZE (mode)) > UNITS_PER_WORD
-	&& (SUBREG_BYTE (x) % UNITS_PER_WORD
-	    || GET_MODE_SIZE (mode) % UNITS_PER_WORD))
+    if ((subreg_byte % UNITS_PER_WORD
+	 + GET_MODE_SIZE (outer_mode)) > UNITS_PER_WORD
+	&& (subreg_byte % UNITS_PER_WORD
+	    || GET_MODE_SIZE (outer_mode) % UNITS_PER_WORD))
 	abort ();
 
   if (WORDS_BIG_ENDIAN)
     word = (GET_MODE_SIZE (inner_mode)
-	    - (SUBREG_BYTE (x) + GET_MODE_SIZE (mode))) / UNITS_PER_WORD;
+	    - (subreg_byte + GET_MODE_SIZE (outer_mode))) / UNITS_PER_WORD;
   else
-    word = SUBREG_BYTE (x) / UNITS_PER_WORD;
+    word = subreg_byte / UNITS_PER_WORD;
   bitpos = word * BITS_PER_WORD;
 
   if (BYTES_BIG_ENDIAN)
     byte = (GET_MODE_SIZE (inner_mode)
-	    - (SUBREG_BYTE (x) + GET_MODE_SIZE (mode))) % UNITS_PER_WORD;
+	    - (subreg_byte + GET_MODE_SIZE (outer_mode))) % UNITS_PER_WORD;
   else
-    byte = SUBREG_BYTE (x) % UNITS_PER_WORD;
+    byte = subreg_byte % UNITS_PER_WORD;
   bitpos += byte * BITS_PER_UNIT;
 
   return bitpos;
 }
 
+/* Given a subreg X, return the bit offset where the subreg begins
+   (counting from the least significant bit of the reg).  */
+
+unsigned int
+subreg_lsb (rtx x)
+{
+  return subreg_lsb_1 (GET_MODE (x), GET_MODE (SUBREG_REG (x)),
+		       SUBREG_BYTE (x));
+}
+
 /* This function returns the regno offset of a subreg expression.
    xregno - A regno of an inner hard subreg_reg (or what will become one).
    xmode  - The mode of xregno.

gcc/simplify-rtx.c

@@ -3379,10 +3379,45 @@ simplify_subreg (enum machine_mode outermode, rtx op,
       res = simplify_subreg (outermode, part, GET_MODE (part), final_offset);
       if (res)
 	return res;
-      /* We can at least simplify it by referring directly to the relevant part.  */
+      /* We can at least simplify it by referring directly to the
+	 relevant part.  */
       return gen_rtx_SUBREG (outermode, part, final_offset);
     }
 
+  /* Optimize SUBREG truncations of zero and sign extended values.  */
+  if ((GET_CODE (op) == ZERO_EXTEND
+       || GET_CODE (op) == SIGN_EXTEND)
+      && GET_MODE_BITSIZE (outermode) < GET_MODE_BITSIZE (innermode))
+    {
+      unsigned int bitpos = subreg_lsb_1 (outermode, innermode, byte);
+
+      /* If we're requesting the lowpart of a zero or sign extension,
+	 there are three possibilities.  If the outermode is the same
+	 as the origmode, we can omit both the extension and the subreg.
+	 If the outermode is not larger than the origmode, we can apply
+	 the truncation without the extension.  Finally, if the outermode
+	 is larger than the origmode, but both are integer modes, we
+	 can just extend to the appropriate mode.  */
+      if (bitpos == 0)
+	{
+	  enum machine_mode origmode = GET_MODE (XEXP (op, 0));
+	  if (outermode == origmode)
+	    return XEXP (op, 0);
+	  if (GET_MODE_BITSIZE (outermode) <= GET_MODE_BITSIZE (origmode))
+	    return simplify_gen_subreg (outermode, XEXP (op, 0),
+					origmode, byte);
+	  if (SCALAR_INT_MODE_P (outermode))
+	    return simplify_gen_unary (GET_CODE (op), outermode,
+				       XEXP (op, 0), origmode);
+	}
+
+      /* A SUBREG resulting from a zero extension may fold to zero if
+	 it extracts higher bits that the ZERO_EXTEND's source bits.  */
+      if (GET_CODE (op) == ZERO_EXTEND
+	  && bitpos >= GET_MODE_BITSIZE (GET_MODE (XEXP (op, 0))))
+	return CONST0_RTX (outermode);
+    }
+
   return NULL_RTX;
 }