[patch] Uniquization of constants at the Tree level

[Eric Botcazou]

Hi,

this is something we've been using in our 4.x compilers for quite some time.
Constant objects (*_CST and CONSTRUCTOR) are uniquized at the RTL level by 
means of output_constant_def but there is no equivalent at the Tree level so, 
if a constant is commited to read-only memory at this level, it might be 
unnecessarily duplicated like in the following artificial C example:

int lookup1 (int i)
{
  int a[] = { 0, 1, 2, 3, 4, 5, 6, 7 };
  return a[i];
}

int lookup2 (int i)
{
  int a[] = { 0, 1, 2, 3, 4, 5, 6, 7 };
  return a[i+1];
}

Compiled at -Os for i686, the assembly file contains:

        .section        .rodata
        .align 4
        .type   C.0.1957, @object
        .size   C.0.1957, 32
C.0.1957:
        .long   0
        .long   1
        .long   2
        .long   3
        .long   4
        .long   5
        .long   6
        .long   7
        .align 4
        .type   C.1.1961, @object
        .size   C.1.1961, 32
C.1.1961:
        .long   0
        .long   1
        .long   2
        .long   3
        .long   4
        .long   5
        .long   6
        .long   7


The patch introduces the counterpart of output_constant_def at the Tree level, 
namely tree_output_constant_def, which returns something directly usable in 
the GENERIC/GIMPLE IL (a VAR_DECL) when passed a valid constant, doing the 
uniquization and output work behind the scene automatically by means of the 
varasm.c uniquization machinery and the varpool.

The new feature is used in gimplify_init_constructor, which eliminates the 
duplication for the above C example, as well as in gnat_gimplify_expr when an 
address of a constant constructor is gimplified (this happens often in Ada).
Other front-ends could presumably use it as well in equivalent cases.

The implementation is straightforward, modulo the following nits:
 - the varasm.c uniquization machinery wasn't updated for VECTOR_CSTs so the 
patch does that,
 - same for CONSTANT_ALIGNMENT of VECTOR_TYPEs for most architectures so the 
patch uses DATA_ALIGNMENT as well,
 - config/arm/unknown-elf.h duplicates a macro of varasm.c and someone thought 
it was clever to undefine the instance in varasm.c... so the patch untangles 
that (no functional changes for the ARM target because of that).

Tested on x86_64-suse-linux and i586-suse-linux with mainline, but a backport 
to our 4.3-based compiler is in use on a wide range of platforms without any 
problems as of this writing.  OK for mainline?


2010-04-25  Eric Botcazou  <ebotcazou@adacore.com>

	Uniquization of constants at the Tree level
	* tree.h (tree_output_constant_def): Declare.
	* gimplify.c (gimplify_init_constructor): When using block copy,
	uniquize the constant constructor on the RHS.
	* varasm.c (IN_NAMED_SECTION): Remove guard.
	(tree_constant_section_name): New static variable.
	(DECL_TREE_LEVEL_CONSTANT_P): New macro.
	(make_decl_rtl): Deal with Tree level constants.
	(assemble_variable): If the underlying symbol is attached to a
	uniquized constant, output the constant.
	(decode_addr_const): Handle special case of INDIRECT_REF.
	(const_hash_1) <VECTOR_CST>: New case.
	(compare_constant) <VECTOR_CST>: Likewise.
	<ADDR_EXPR>: Deal with LABEL_REFs.
	(copy_constant) <VECTOR_CST>: New case.
	(get_constant_alignment): Delete.
	(get_constant_section): Add ALIGN parameter and simplify.
	(build_constant_desc): Build a VAR_DECL and attach it to the symbol.
	(assemble_constant_contents): Use the expression of the VAR_DECL.
	(output_constant_def_contents): Use the alignment of the VAR_DECL.
	(tree_output_constant_def): New global function.
	(mark_constant): Use the expression of the VAR_DECL.
	(init_varasm_once): Initialize tree_constant_section_name.
	(place_block_symbol): Use the alignment of the VAR_DECL and the size
	of its expression.
	(output_object_block): Likewise and assemble the expression.
	* config/arm/unknown-elf.h (IN_NAMED_SECTION): Rename to...
	(IN_NAMED_SECTION_P): ...this.
	(ASM_OUTPUT_ALIGNED_BSS): Adjust for above renaming.
	(ASM_OUTPUT_ALIGNED_DECL_LOCAL): Likewise.
ada/
	* gcc-interface/trans.c (gnat_gimplify_expr) <ADDR_EXPR>: Uniquize
	constant constructors before taking their address.


-- 
Eric Botcazou
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