[RFC 4/5] Handle constant-pool entries

[Alan Lawrence <alan dot lawrence at arm dot com>]

This makes SRA replace loads of records/arrays from constant pool entries,
with elementwise assignments of the constant values, hence, overcoming the
fundamental problem in PR/63679.

As a first pass, the approach I took was to look for constant-pool loads as
we scanned through other accesses, and add them as candidates there; to build a
constant replacement_decl for any such accesses in completely_scalarize; and to
use any existing replacement_decl rather than creating a variable in
create_access_replacement. (I did try using CONSTANT_CLASS_P in the latter, but
that does not allow addresses of labels, which can still end up in the constant
pool.)

Feedback as to the approach or how it might be better structured / fitted into
SRA, is solicited ;).

Bootstrapped + check-gcc on x86-none-linux-gnu, aarch64-none-linux-gnu and
arm-none-linux-gnueabihf, including with the next patch (rfc), which greatly increases the number of testcases in which this code is exercised!

Have also verified that the ssa-dom-cse-2.c scan-tree-dump test passes (using a stage 1 compiler only, without execution) on alpha, hppa, powerpc, sparc, avr, and sh.

gcc/ChangeLog:

	* tree-sra.c (create_access): Scan for uses of constant pool and add
	to candidates.
	(subst_initial): New.
	(scalarize_elem): Build replacement_decl using subst_initial.
	(create_access_replacement): Use replacement_decl if set.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/ssa-dom-cse-2.c: Remove xfail, add --param
	sra-max-scalarization-size-Ospeed.
---
 gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-cse-2.c |  7 +---
 gcc/tree-sra.c                                | 56 +++++++++++++++++++++++++--
 2 files changed, 55 insertions(+), 8 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-cse-2.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-cse-2.c
index 9eccdc9..b13d583 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-cse-2.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-cse-2.c
@@ -1,5 +1,5 @@
 /* { dg-do compile } */
-/* { dg-options "-O3 -fno-tree-fre -fno-tree-pre -fdump-tree-optimized" } */
+/* { dg-options "-O3 -fno-tree-fre -fno-tree-pre -fdump-tree-optimized --param sra-max-scalarization-size-Ospeed=32" } */
 
 int
 foo ()
@@ -17,7 +17,4 @@ foo ()
 /* After late unrolling the above loop completely DOM should be
    able to optimize this to return 28.  */
 
-/* See PR63679 and PR64159, if the target forces the initializer to memory then
-   DOM is not able to perform this optimization.  */
-
-/* { dg-final { scan-tree-dump "return 28;" "optimized" { xfail aarch64*-*-* alpha*-*-* hppa*-*-* powerpc*-*-* sparc*-*-* s390*-*-* } } } */
+/* { dg-final { scan-tree-dump "return 28;" "optimized" } } */
diff --git a/gcc/tree-sra.c b/gcc/tree-sra.c
index af35fcc..a3ff2df 100644
--- a/gcc/tree-sra.c
+++ b/gcc/tree-sra.c
@@ -865,6 +865,17 @@ create_access (tree expr, gimple stmt, bool write)
   else
     ptr = false;
 
+  /* FORNOW: scan for uses of constant pool as we go along.  */
+  if (TREE_CODE (base) == VAR_DECL && DECL_IN_CONSTANT_POOL (base)
+      && !bitmap_bit_p (candidate_bitmap, DECL_UID (base)))
+    {
+      gcc_assert (!write);
+      bitmap_set_bit (candidate_bitmap, DECL_UID (base));
+      tree_node **slot = candidates->find_slot_with_hash (base, DECL_UID (base),
+							  INSERT);
+      *slot = base;
+    }
+
   if (!DECL_P (base) || !bitmap_bit_p (candidate_bitmap, DECL_UID (base)))
     return NULL;
 
@@ -1025,6 +1036,37 @@ completely_scalarize (tree base, tree decl_type, HOST_WIDE_INT offset, tree ref)
     }
 }
 
+static tree
+subst_initial (tree expr, tree var)
+{
+  if (TREE_CODE (expr) == VAR_DECL)
+    {
+      gcc_assert (DECL_IN_CONSTANT_POOL (expr));
+      gcc_assert (expr == var);
+      return DECL_INITIAL (expr);
+    }
+  if (TREE_CODE (expr) == COMPONENT_REF)
+    {
+      gcc_assert (TREE_CODE (TREE_OPERAND (expr, 1)) == FIELD_DECL);
+      gcc_assert (TREE_OPERAND (expr, 2) == NULL_TREE);
+      return fold_build3 (COMPONENT_REF, TREE_TYPE (expr),
+			  subst_initial (TREE_OPERAND (expr, 0), var),
+			  TREE_OPERAND (expr, 1),
+			  NULL_TREE);
+    }
+  if (TREE_CODE (expr) == ARRAY_REF)
+    {
+      gcc_assert (TREE_OPERAND (expr, 2) == NULL_TREE);
+      gcc_assert (TREE_OPERAND (expr, 3) == NULL_TREE);
+      return fold (build4 (ARRAY_REF, TREE_TYPE (expr),
+			   subst_initial (TREE_OPERAND (expr, 0), var),
+			   TREE_OPERAND (expr, 1),
+			   NULL_TREE,
+			   NULL_TREE));
+    }
+  gcc_unreachable ();
+}
+
 static void
 scalarize_elem (tree base, HOST_WIDE_INT pos, HOST_WIDE_INT size,
 		 tree ref, tree type)
@@ -1033,6 +1075,9 @@ scalarize_elem (tree base, HOST_WIDE_INT pos, HOST_WIDE_INT size,
   {
     struct access *access = create_access_1 (base, pos, size);
     access->expr = ref;
+    if (TREE_CODE (base) == VAR_DECL
+	&& DECL_IN_CONSTANT_POOL (base))
+      access->replacement_decl = subst_initial (ref, base);
     access->type = type;
     access->grp_total_scalarization = 1;
     /* Accesses for intraprocedural SRA can have their stmt NULL.  */
@@ -2038,14 +2083,19 @@ sort_and_splice_var_accesses (tree var)
 }
 
 /* Create a variable for the given ACCESS which determines the type, name and a
-   few other properties.  Return the variable declaration and store it also to
-   ACCESS->replacement.  */
+   few other properties.  Return the variable declaration.  */
 
 static tree
 create_access_replacement (struct access *access)
 {
-  tree repl;
+  if (access->replacement_decl)
+    {
+      gcc_assert (!access->write);
+      gcc_assert (is_gimple_reg_type (TREE_TYPE (access->expr)));
+      return access->replacement_decl;
+    }
 
+  tree repl;
   if (access->grp_to_be_debug_replaced)
     {
       repl = create_tmp_var_raw (access->type);
-- 
1.8.3