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Bug 14705 - [tree-ssa] more alias analysis needed!?
Summary: [tree-ssa] more alias analysis needed!?
Alias: None
Product: gcc
Classification: Unclassified
Component: tree-optimization (show other bugs)
Version: tree-ssa
: P2 enhancement
Target Milestone: 4.2.0
Assignee: Not yet assigned to anyone
Keywords: alias, missed-optimization, TREE
Depends on:
Reported: 2004-03-24 03:44 UTC by Kazu Hirata
Modified: 2006-02-16 21:17 UTC (History)
2 users (show)

See Also:
Known to work:
Known to fail:
Last reconfirmed: 2005-12-07 03:16:12


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Description Kazu Hirata 2004-03-24 03:44:38 UTC
The second "if" statement is not optimized away until CSE.

/* Reduced from reload_combine() in postreload.c.  */

struct s {
  int c;
  struct s *next;

int global;

extern void bar (void);

foo (struct s *p)
  for (; p; p = p->next)
      if (p->c != 0)

      if (p->c == 0)
	bar ();

To give you sense of where this comes from,
think of the first "if" as !INSN_P and
think of the second one as INSN_P with bar() being single_set_2.
Comment 1 Andrew Pinski 2004-03-24 04:34:59 UTC
Variable: p, UID 1, type memory tag: TMT.3, default def: p_5
Variable: TMT.3, UID 5, is global, call clobbered, may aliases: { global }
Comment 2 Kazu Hirata 2004-03-24 17:30:32 UTC
Reduced to:

struct s {
  int c;
  struct s *next;

int global;

extern void bar (void);

foo (struct s *p)
  if (p->c != 0)

  if (p->c == 0)
    bar ();
Comment 3 Kazu Hirata 2004-03-24 17:32:38 UTC
Another similar testcase reduced from web.c.

extern void abort (void) __attribute__ ((__noreturn__));

struct s {
  int a;
  int b;

bar (struct s *p)
  if (p->a)
    abort ();

  p->b = 0;

  if (p->a)
    abort ();
Comment 4 Andrew Pinski 2004-03-24 17:36:29 UTC
The reduced case from web.c is also PR 13761.
Comment 5 Steven Bosscher 2004-12-30 14:05:34 UTC
Looks like food for DannyB.
Comment 6 Kazu Hirata 2005-05-07 15:03:26 UTC
FWIW, here is the last SSA form I get with vops.

foo (p)
  int D.1241;
  int global.0;
  int D.1239;

<bb 0>:
  #   VUSE <global_3>;
  D.1239_2 = p_1->c;
  if (D.1239_2 != 0) goto <L2>; else goto <L0>;

  #   VUSE <global_3>;
  global.0_4 = global;
  D.1241_5 = global.0_4 + 1;
  #   global_6 = V_MUST_DEF <global_3>;
  global = D.1241_5;
  #   VUSE <global_6>;
  D.1239_7 = p_1->c;
  if (D.1239_7 == 0) goto <L1>; else goto <L2>;

  #   global_8 = V_MAY_DEF <global_6>;
  bar () [tail call];


Comment 7 Andrew Pinski 2006-02-16 21:17:14 UTC
Comment #0 and #2 are fixed now by:
2006-02-16  Daniel Berlin <>

        * tree-ssa-operands.c (access_can_touch_variable): Fix typo in
         previous change.
2006-02-15 Daniel Berlin  <>

        * tree.c (init_ttree): Add STRUCT_FIELD_TAG handling.
        (tree_code_size): Ditto.
        * tree.h (struct tree_memory_tag): Remove parent_var.
        (struct tree_struct_field_tag): New.
        (SFT_OFFSET): New.
        (SFT_SIZE): New.
        (union tree_node): Add sft member.
        * tree-ssa-alias.c (get_tmt_for): Don't handle TYPE_READONLY
        specially here.
        (create_sft): Add size and offset argument, set SFT_OFFSET and
        (create_overlap_variables_for): Update for SFT_OFFSET/SFT_SIZE.
        * treestruct.def: Add TS_STRUCT_FIELD_TAG.
        * tree-flow-inline.h (get_subvar_at): Update for
        (var_can_have_subvars): Ditto.
        (overlap_subvar): Ditto.
        * print-tree.c (print_node): Print out interesting things for
        * tree-flow.h (struct subvar): Remove offset and size members.
        * tree-ssa-operands.c (get_expr_operands): Update for
        get_indirect_ref_operands changes.
        (get_indirect_ref_operands): Call add_virtual_operand instead of
        add_stmt_operand.  Only recurse on base var if requested.
        (access_can_touch_variable): New function.
        (add_stmt_operand): Split virtual operand handling into ...
        (add_virtual_operand): Here.  Add offset, size, and for_clobber
        arguments.  Prune alias sets.
        (add_call_clobber_ops): Call add_virtual_operand.

Comment #3 as mentioned it PR 13761.