Take the following code, we should not abort: int func_pure (void); void func_other (int); int global_int; int func_pure (void) { return global_int; } void func_other (int a) { global_int = a + 1; } int f(void) { int a; a = func_pure(); func_other (a); a = func_pure (); // We are removing this function call return a; } void abort (void); int main(void) { global_int = 10; if (f() != 11) abort (); }
This is a DOM bug. Comment above lookup_avail_expr says: NOTE: This function assumes that STMT is a MODIFY_EXPR node that contains no CALL_EXPR on its RHS and makes no volatile nor aliased references. */ and I have no reason not to believe this, if not perhaps for const calls, then certainly for pure calls. For pure calls the code would need to invalidate all recorded pure calls whenever a stmt might modify global state. But optimize_stmt/eliminate_redundant_computations will happily call lookup_avail_expr with a CALL_EXPR on the RHS, because const/pure calls don't have TREE_SIDE_EFFECTS set. I have tried: --- tree-ssa-dom.c.jj 2005-02-17 20:02:58.000000000 +0100 +++ tree-ssa-dom.c 2005-02-22 14:40:14.912216290 +0100 @@ -2964,7 +2964,8 @@ optimize_stmt (struct dom_walk_data *wal || (TREE_CODE (stmt) == MODIFY_EXPR && ! TREE_SIDE_EFFECTS (TREE_OPERAND (stmt, 1))) || TREE_CODE (stmt) == COND_EXPR - || TREE_CODE (stmt) == SWITCH_EXPR)); + || TREE_CODE (stmt) == SWITCH_EXPR) + && get_call_expr_in (stmt) == NULL); if (may_optimize_p) may_have_exposed_new_symbols which fixes this, but it seems no other tree-SSA optimization is able to optimize say extern int foo (void) __attribute__((pure)); ... a = foo (); a += foo (); into a = 2 * foo ();, fortunately RTL optimizations optimize that. If const calls are actually handled correctly by lookup_avail_expr, then the comment above it ought to be changed and we could only avoid optimizing DECL_IS_PURE calls in addition to TREE_SIDE_EFFECTS rhs' in optimize_stmt.
Note this was cause by: 2005-01-26 Diego Novillo <dnovillo@redhat.com> PR tree-optimization/19633 * tree-ssa-alias.c (ptr_is_dereferenced_by): Also handle CALL_EXPRs. (maybe_create_global_var): Do not create .GLOBAL_VAR if there are no call-clobbered variables. * tree-outof-ssa.c (check_replaceable): Return false for calls with side-effects.
Also note that DOM is doing the "correct" thing as the DOM does not know that func_other could change the value of func_pure.
> For pure calls the code would need to invalidate > all recorded pure calls whenever a stmt might modify global state. Yes this was done with GLOBAL_VAR before Diego's patch but now it is not done with anything else. This is basically the same issue as PR 20100.
Testing patch.
Subject: Bug 20115 CVSROOT: /cvs/gcc Module name: gcc Changes by: dnovillo@gcc.gnu.org 2005-02-23 05:08:33 Modified files: gcc : ChangeLog tree-optimize.c tree-pass.h tree-ssa-alias.c tree-ssa-operands.c tree-ssa.c gcc/testsuite : ChangeLog gcc/testsuite/gcc.dg/tree-ssa: 20040517-1.c Added files: gcc/testsuite/gcc.dg: pr20100.c pr20115-1.c pr20115.c Log message: PR tree-optimization/20100 PR tree-optimization/20115 * tree-optimize.c (init_tree_optimization_passes): Remove pass_maybe_create_global_var. * tree-pass.h (pass_maybe_create_global_var): Remove. * tree-ssa-alias.c (aliases_computed_p): Declare. (struct alias_info): Add field NUM_PURE_CONST_CALLS_FOUND. (count_calls_and_maybe_create_global_var): Remove. (pass_maybe_create_global_var): Remove. (init_alias_info): Do not declare aliases_computed_p. (maybe_create_global_var): If the function contains no call-clobbered variables and a mix of pure/const and regular function calls, create .GLOBAL_VAR. Mark all call-clobbered variables for renaming. (merge_pointed_to_info): Update comment. (add_pointed_to_var): Likewise. (is_escape_site): Likewise. Accept struct alias_info * instead of size_t *. Update all users. Update AI->NUM_CALLS_FOUND and AI->NUM_PURE_CONST_CALLS_FOUND as necessary. * tree-ssa-operands.c (get_call_expr_operands): If ALIASES_COMPUTED_P is false, do not add call-clobbering operands. * tree-ssa.c (init_tree_ssa): Set ALIASES_COMPUTED_P to false. (delete_tree_ssa): Likewise. testsuite/ChangeLog PR tree-optimization/20100 PR tree-optimization/20115 * gcc.dg/pr20115.c: New test. * gcc.dg/pr20115-1.c: New test. * gcc.dg/pr20100.c: New test. * gcc.dg/tree-ssa/20040517-1.c: Expect virtual operands for call-clobbered variables after alias1. Patches: http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&r1=2.7566&r2=2.7567 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/tree-optimize.c.diff?cvsroot=gcc&r1=2.73&r2=2.74 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/tree-pass.h.diff?cvsroot=gcc&r1=2.26&r2=2.27 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/tree-ssa-alias.c.diff?cvsroot=gcc&r1=2.70&r2=2.71 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/tree-ssa-operands.c.diff?cvsroot=gcc&r1=2.62&r2=2.63 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/tree-ssa.c.diff?cvsroot=gcc&r1=2.77&r2=2.78 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&r1=1.5071&r2=1.5072 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/gcc.dg/pr20100.c.diff?cvsroot=gcc&r1=NONE&r2=1.1 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/gcc.dg/pr20115-1.c.diff?cvsroot=gcc&r1=NONE&r2=1.1 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/gcc.dg/pr20115.c.diff?cvsroot=gcc&r1=NONE&r2=1.1 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/gcc.dg/tree-ssa/20040517-1.c.diff?cvsroot=gcc&r1=1.3&r2=1.4
http://gcc.gnu.org/ml/gcc-patches/2005-02/msg01387.html