| Message ID | 002a01d773df$202b5150$6081f3f0$@nextmovesoftware.com |
|---|---|
| State | New |
| Headers | show |
| Series | PR tree-optimization/38943: Preserve trapping instructions with -fnon-call-exceptions | expand |
On Thu, Jul 8, 2021 at 11:54 AM Roger Sayle <roger@nextmovesoftware.com> wrote: > > > This patch addresses PR tree-optimization/38943 where gcc may optimize > away trapping instructions even when -fnon-call-exceptions is specified. > Interestingly this only affects the C compiler (when -fexceptions is not > specified) as g++ (or -fexceptions) supports C++-style exception handling, > where -fnon-call-exceptions triggers the stmt_could_throw_p machinery. > Without -fexceptions, trapping instructions aren't always considered > visible side-effects. But -fnon-call-exceptions without -fexceptions doesn't make much sense, does it? I see the testcase behaves correctly when -fexceptions is also specified. The call vanishes in DCE because stmt_could_throw_p starts with bool stmt_could_throw_p (function *fun, gimple *stmt) { if (!flag_exceptions) return false; the documentation of -fnon-call-exceptions says Generate code that allows trapping instructions to throw exceptions. so either the above check is wrong or -fnon-call-exceptions should imply -fexceptions (or we should diagnose missing -fexceptions) > > This patch fixes this in two place. Firstly, gimple_has_side_effects > is tweaked such that gimple_could_trap_p is considered a side-effect > if the current function can throw non-call exceptions. But exceptions are not considered side-effects - they are explicit in the IL and thus passes are supposed to check for those and preserve dead (externally) throwing stmts if not told otherwise (flag_delete_dead_exceptions). > And secondly, > check_stmt in ipa-pure-const.c is tweaked such that a function > containing a trapping statement is considered to have a side-effect > with -fnon-call-exceptions, and therefore cannot be pure or const. EH is orthogonal to pure/const, so I think that's wrong. > Calling gimple_could_trap_p (which previously took a non-const gimple) > from gimple_has_side_effects (which takes a const gimple) required > improving the const-safety of gimple_could_trap_p (a relatively minor > tweak) and its prototypes. Hopefully this is considered a clean-up/ > improvement. Yeah, even an obvious one I think - you can push that independently. > This patch has been tested on x86_64-pc-linux-gnu with a "make > bootstrap" and "make -k check" with no new failures. This should > be relatively safe, as there are no changes in behaviour unless > the user explicitly specifies -fnon-call-exceptions, when the C > compiler then behaves more like the C++/Ada compiler. > > Ok for mainline? > > > 2021-07-08 Roger Sayle <roger@nextmovesoftware.com> > > gcc/ChangeLog > PR tree-optimization/38943 > * gimple.c (gimple_has_side_effects): Consider trapping to > be a side-effect when -fnon-call-exceptions is specified. > (gimple_coult_trap_p_1): Make S argument a "const gimple*". > Preserve constness in call to gimple_asm_volatile_p. > (gimple_could_trap_p): Make S argument a "const gimple*". > * gimple.h (gimple_could_trap_p_1, gimple_could_trap_p): > Update function prototypes. > * ipa-pure-const.c (check_stmt): When the current function > can throw non-call exceptions, a trapping statement should > be considered a side-effect, so the function is neither > const nor pure. > > gcc/testsuite/ChangeLog > PR tree-optimization/38943 > * gcc.dg/pr38943.c: New test case. > > Roger > -- > Roger Sayle > NextMove Software > Cambridge, UK >
> This patch has been tested on x86_64-pc-linux-gnu with a "make > bootstrap" and "make -k check" with no new failures. This should > be relatively safe, as there are no changes in behaviour unless > the user explicitly specifies -fnon-call-exceptions, when the C > compiler then behaves more like the C++/Ada compiler. I think this will pessimize Ada, which defaults to -fnon-call-exceptions but where we do *not* want to preserve trapping instructions just because they may trap (i.e. -fdelete-dead-exceptions is enabled by default). And, as noticed by Richard, EH is orthogonal to side effects and pure/const.
diff --git a/gcc/gimple.c b/gcc/gimple.c index f1044e9..4b150b0 100644 --- a/gcc/gimple.c +++ b/gcc/gimple.c @@ -2090,7 +2090,8 @@ gimple_move_vops (gimple *new_stmt, gimple *old_stmt) statement to have side effects if: - It is a GIMPLE_CALL not marked with ECF_PURE or ECF_CONST. - - Any of its operands are marked TREE_THIS_VOLATILE or TREE_SIDE_EFFECTS. */ + - Any of its operands are marked TREE_THIS_VOLATILE or TREE_SIDE_EFFECTS. + - It may trap and -fnon-call-exceptions has been specified. */ bool gimple_has_side_effects (const gimple *s) @@ -2108,6 +2109,10 @@ gimple_has_side_effects (const gimple *s) && gimple_asm_volatile_p (as_a <const gasm *> (s))) return true; + if (cfun->can_throw_non_call_exceptions + && gimple_could_trap_p (s)) + return true; + if (is_gimple_call (s)) { int flags = gimple_call_flags (s); @@ -2129,7 +2134,7 @@ gimple_has_side_effects (const gimple *s) S is a GIMPLE_ASSIGN, the LHS of the assignment is also checked. */ bool -gimple_could_trap_p_1 (gimple *s, bool include_mem, bool include_stores) +gimple_could_trap_p_1 (const gimple *s, bool include_mem, bool include_stores) { tree t, div = NULL_TREE; enum tree_code op; @@ -2146,7 +2151,7 @@ gimple_could_trap_p_1 (gimple *s, bool include_mem, bool include_stores) switch (gimple_code (s)) { case GIMPLE_ASM: - return gimple_asm_volatile_p (as_a <gasm *> (s)); + return gimple_asm_volatile_p (as_a <const gasm *> (s)); case GIMPLE_CALL: t = gimple_call_fndecl (s); @@ -2192,7 +2197,7 @@ gimple_could_trap_p_1 (gimple *s, bool include_mem, bool include_stores) /* Return true if statement S can trap. */ bool -gimple_could_trap_p (gimple *s) +gimple_could_trap_p (const gimple *s) { return gimple_could_trap_p_1 (s, true, true); } diff --git a/gcc/gimple.h b/gcc/gimple.h index e7dc2a4..1a2e120 100644 --- a/gcc/gimple.h +++ b/gcc/gimple.h @@ -1601,8 +1601,8 @@ void gimple_set_lhs (gimple *, tree); gimple *gimple_copy (gimple *); void gimple_move_vops (gimple *, gimple *); bool gimple_has_side_effects (const gimple *); -bool gimple_could_trap_p_1 (gimple *, bool, bool); -bool gimple_could_trap_p (gimple *); +bool gimple_could_trap_p_1 (const gimple *, bool, bool); +bool gimple_could_trap_p (const gimple *); bool gimple_assign_rhs_could_trap_p (gimple *); extern void dump_gimple_statistics (void); unsigned get_gimple_rhs_num_ops (enum tree_code); diff --git a/gcc/ipa-pure-const.c b/gcc/ipa-pure-const.c index f045108..436cbcd 100644 --- a/gcc/ipa-pure-const.c +++ b/gcc/ipa-pure-const.c @@ -765,6 +765,14 @@ check_stmt (gimple_stmt_iterator *gsip, funct_state local, bool ipa) if (dump_file) fprintf (dump_file, " Volatile stmt is not const/pure\n"); } + else if (cfun->can_throw_non_call_exceptions + && gimple_code (stmt) != GIMPLE_CALL + && gimple_could_trap_p (stmt)) + { + local->pure_const_state = IPA_NEITHER; + if (dump_file) + fprintf (dump_file, " Trapping stmt is not const/pure\n"); + } /* Look for loads and stores. */ walk_stmt_load_store_ops (stmt, local,
This patch addresses PR tree-optimization/38943 where gcc may optimize away trapping instructions even when -fnon-call-exceptions is specified. Interestingly this only affects the C compiler (when -fexceptions is not specified) as g++ (or -fexceptions) supports C++-style exception handling, where -fnon-call-exceptions triggers the stmt_could_throw_p machinery. Without -fexceptions, trapping instructions aren't always considered visible side-effects. This patch fixes this in two place. Firstly, gimple_has_side_effects is tweaked such that gimple_could_trap_p is considered a side-effect if the current function can throw non-call exceptions. And secondly, check_stmt in ipa-pure-const.c is tweaked such that a function containing a trapping statement is considered to have a side-effect with -fnon-call-exceptions, and therefore cannot be pure or const. Calling gimple_could_trap_p (which previously took a non-const gimple) from gimple_has_side_effects (which takes a const gimple) required improving the const-safety of gimple_could_trap_p (a relatively minor tweak) and its prototypes. Hopefully this is considered a clean-up/ improvement. This patch has been tested on x86_64-pc-linux-gnu with a "make bootstrap" and "make -k check" with no new failures. This should be relatively safe, as there are no changes in behaviour unless the user explicitly specifies -fnon-call-exceptions, when the C compiler then behaves more like the C++/Ada compiler. Ok for mainline? 2021-07-08 Roger Sayle <roger@nextmovesoftware.com> gcc/ChangeLog PR tree-optimization/38943 * gimple.c (gimple_has_side_effects): Consider trapping to be a side-effect when -fnon-call-exceptions is specified. (gimple_coult_trap_p_1): Make S argument a "const gimple*". Preserve constness in call to gimple_asm_volatile_p. (gimple_could_trap_p): Make S argument a "const gimple*". * gimple.h (gimple_could_trap_p_1, gimple_could_trap_p): Update function prototypes. * ipa-pure-const.c (check_stmt): When the current function can throw non-call exceptions, a trapping statement should be considered a side-effect, so the function is neither const nor pure. gcc/testsuite/ChangeLog PR tree-optimization/38943 * gcc.dg/pr38943.c: New test case. Roger -- Roger Sayle NextMove Software Cambridge, UK /* { dg-do compile } */ /* { dg-options "-O2 -fnon-call-exceptions -fdump-tree-optimized" } */ float __attribute__((noinline)) foo (float x) { return 2.0f / x; }; void bar() { foo (3.0f); } /* { dg-final { scan-tree-dump "foo \\(3" "optimized" } } */