On Thu, Jul 8, 2021 at 11:54 AM Roger Sayle <ro...@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 <ro...@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
>
