RFA: Make s390 use an UNSPEC instead of (const (minus ...))

[Richard Sandiford <rdsandiford at googlemail dot com>]

This patch makes the s390 port use UNSPECs instead of (const (minus ...)).
It's part of my ongoing quest to enforce a grammar for CONSTs.  See:

    http://gcc.gnu.org/ml/gcc-patches/2008-10/msg00339.html

for details.

The port only uses (const (minus ...)) to represent the difference
between two pool labels.  It originally allowed:

    (const (minus (label_ref ...) (label_ref ...)))

at all times, but this caused PR 26018 (which is similiar to the
CRIS problem I'm trying to fix).  It was therefore necessary to
add a state variable to say whether the address routines are
being called after the start of md_reorg.

I think using an UNSPEC is a little cleaner than a state variable.
Not all label differences are valid even when the variable is true;
we really only want to allow specific differences that have already
been "blessed" by the pool-layout code.  UNSPECs give exactly the
kind of target-specific control we need.

Tested by comparing the before and after assembly output for
gcc.c-torture, gcc.dg and g++.dg on s390x-linux-gnu, using the
options:

  {-O0,-O2} x {-fno-pic,-fpic} x {-m31,-m64}

There were no differences.  OK to install?

Richard


gcc/
	* config/s390/s390.md (UNSPEC_POOL_OFFSET): New constant.
	* config/s390/s390.c (machine_function): Remove
	decomposed_literal_pool_addresses_ok_p.
	(s390_decompose_address): Check for UNSPEC_POOL_OFFSET instead of
	the difference of two labels.
	(s390_output_addr_const_extra): Handle UNSPEC_POOL_OFFSET.
	(s390_pool_offset): New function.
	(s390_find_constant, s390_find_execute, s390_dump_pool): Use it.
	(s390_reorg): Don't set decomposed_literal_pool_addresses_ok_p.

Index: gcc/config/s390/s390.md
===================================================================
--- gcc/config/s390/s390.md	2008-10-10 10:33:42.000000000 +0100
+++ gcc/config/s390/s390.md	2008-10-10 10:33:48.000000000 +0100
@@ -68,6 +68,7 @@ (define_constants
    ; GOT/PLT and lt-relative accesses
    (UNSPEC_LTREL_OFFSET		100)
    (UNSPEC_LTREL_BASE		101)
+   (UNSPEC_POOL_OFFSET		102)
    (UNSPEC_GOTENT		110)
    (UNSPEC_GOT			111)
    (UNSPEC_GOTOFF		112)
Index: gcc/config/s390/s390.c
===================================================================
--- gcc/config/s390/s390.c	2008-08-31 11:45:07.000000000 +0100
+++ gcc/config/s390/s390.c	2008-10-10 10:33:21.000000000 +0100
@@ -316,9 +316,6 @@ struct machine_function GTY(())
   /* True if we may need to perform branch splitting.  */
   bool split_branches_pending_p;
 
-  /* True during final stage of literal pool processing.  */
-  bool decomposed_literal_pool_addresses_ok_p;
-
   /* Some local-dynamic TLS symbol name.  */
   const char *some_ld_name;
 
@@ -1955,15 +1952,10 @@ s390_decompose_address (rtx addr, struct
 	  ;
         }
 
-      /* Accept chunkified literal pool symbol references.  */
-      else if (cfun && cfun->machine
-	       && cfun->machine->decomposed_literal_pool_addresses_ok_p
-	       && GET_CODE (disp) == MINUS
-               && GET_CODE (XEXP (disp, 0)) == LABEL_REF
-               && GET_CODE (XEXP (disp, 1)) == LABEL_REF)
-        {
-	  ;
-        }
+      /* Accept pool label offsets.  */
+      else if (GET_CODE (disp) == UNSPEC
+	       && XINT (disp, 1) == UNSPEC_POOL_OFFSET)
+	;
 
       /* Accept literal pool references.  */
       else if (GET_CODE (disp) == UNSPEC
@@ -4909,6 +4901,14 @@ s390_output_addr_const_extra (FILE *file
 	return true;
       }
 
+  if (GET_CODE (x) == UNSPEC && XVECLEN (x, 0) == 2)
+    switch (XINT (x, 1))
+      {
+      case UNSPEC_POOL_OFFSET:
+	x = gen_rtx_MINUS (GET_MODE (x), XVECEXP (x, 0, 0), XVECEXP (x, 0, 1));
+	output_addr_const (file, x);
+	return true;
+      }
   return false;
 }
 
@@ -5823,6 +5823,20 @@ s390_add_constant (struct constant_pool 
     }
 }
 
+/* Return an rtx that represents the offset of X from the start of
+   pool POOL.  */
+
+static rtx
+s390_pool_offset (struct constant_pool *pool, rtx x)
+{
+  rtx label;
+
+  label = gen_rtx_LABEL_REF (GET_MODE (x), pool->label);
+  x = gen_rtx_UNSPEC (GET_MODE (x), gen_rtvec (2, x, label),
+		      UNSPEC_POOL_OFFSET);
+  return gen_rtx_CONST (GET_MODE (x), x);
+}
+
 /* Find constant VAL of mode MODE in the constant pool POOL.
    Return an RTX describing the distance from the start of
    the pool to the location of the new constant.  */
@@ -5832,7 +5846,6 @@ s390_find_constant (struct constant_pool
 		    enum machine_mode mode)
 {
   struct constant *c;
-  rtx offset;
   int i;
 
   for (i = 0; i < NR_C_MODES; i++)
@@ -5846,10 +5859,7 @@ s390_find_constant (struct constant_pool
 
   gcc_assert (c);
 
-  offset = gen_rtx_MINUS (Pmode, gen_rtx_LABEL_REF (Pmode, c->label),
-                                 gen_rtx_LABEL_REF (Pmode, pool->label));
-  offset = gen_rtx_CONST (Pmode, offset);
-  return offset;
+  return s390_pool_offset (pool, gen_rtx_LABEL_REF (Pmode, c->label));
 }
 
 /* Check whether INSN is an execute.  Return the label_ref to its
@@ -5897,7 +5907,6 @@ s390_add_execute (struct constant_pool *
 s390_find_execute (struct constant_pool *pool, rtx insn)
 {
   struct constant *c;
-  rtx offset;
 
   for (c = pool->execute; c != NULL; c = c->next)
     if (INSN_UID (insn) == INSN_UID (c->value))
@@ -5905,10 +5914,7 @@ s390_find_execute (struct constant_pool 
 
   gcc_assert (c);
 
-  offset = gen_rtx_MINUS (Pmode, gen_rtx_LABEL_REF (Pmode, c->label),
-				 gen_rtx_LABEL_REF (Pmode, pool->label));
-  offset = gen_rtx_CONST (Pmode, offset);
-  return offset;
+  return s390_pool_offset (pool, gen_rtx_LABEL_REF (Pmode, c->label));
 }
 
 /* For an execute INSN, extract the execute target template.  */
@@ -5989,11 +5995,7 @@ s390_dump_pool (struct constant_pool *po
 	    && GET_CODE (XEXP (value, 0)) == UNSPEC
 	    && XINT (XEXP (value, 0), 1) == UNSPEC_LTREL_OFFSET
 	    && XVECLEN (XEXP (value, 0), 0) == 1)
-	  {
-	    value = gen_rtx_MINUS (Pmode, XVECEXP (XEXP (value, 0), 0, 0),
-				   gen_rtx_LABEL_REF (VOIDmode, pool->label));
-	    value = gen_rtx_CONST (VOIDmode, value);
-	  }
+	  value = s390_pool_offset (pool, XVECEXP (XEXP (value, 0), 0, 0));
 
 	insn = emit_label_after (c->label, insn);
 	INSN_ADDRESSES_NEW (insn, -1);
@@ -9598,9 +9600,6 @@ s390_reorg (void)
      machine_dependent_reorg might confuse insn length counts.  */
   split_all_insns_noflow ();
 
-  /* From here on decomposed literal pool addresses must be accepted.  */
-  cfun->machine->decomposed_literal_pool_addresses_ok_p = true;
-
   /* Install the main literal pool and the associated base
      register load insns.