[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-02-20 15:52 --- Note the testcase in comment 1 is not fixed yet, see PR 20100. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
--- Additional Comments From jakub at gcc dot gnu dot org 2005-02-19 09:33 --- Fixed. -- What|Removed |Added Status|NEW |RESOLVED Resolution||FIXED http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
--- Additional Comments From cvs-commit at gcc dot gnu dot org 2005-02-19 09:26 --- Subject: Bug 19828 CVSROOT:/cvs/gcc Module name:gcc Changes by: [EMAIL PROTECTED] 2005-02-19 09:26:09 Modified files: gcc: ChangeLog tree-ssa-loop-im.c gcc/testsuite : ChangeLog Added files: gcc/testsuite/gcc.c-torture/execute: 20050218-1.c gcc/testsuite/gcc.dg/tree-ssa: loop-7.c Log message: PR tree-optimization/19828 * tree-ssa-loop-im.c: Add a TODO comment. (movement_possibility): Return MOVE_PRESERVE_EXECUTION for calls without side-effects. * gcc.dg/tree-ssa/loop-7.c: New test. * gcc.c-torture/execute/20050218-1.c: New test. Patches: http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&r1=2.7535&r2=2.7536 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/tree-ssa-loop-im.c.diff?cvsroot=gcc&r1=2.27&r2=2.28 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&r1=1.5052&r2=1.5053 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/gcc.c-torture/execute/20050218-1.c.diff?cvsroot=gcc&r1=NONE&r2=1.1 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/gcc.dg/tree-ssa/loop-7.c.diff?cvsroot=gcc&r1=NONE&r2=1.1 -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
--- Additional Comments From rakdver at gcc dot gnu dot org 2005-02-18 16:16 --- OK, I agree that definition of ``pure'' needs to be changed in order to be useful (and to match the expectations); obviously, any function that is not total does not match the current definition. What I find somewhat troublesome is that the "upgraded" definition of pure puts some of obligation on user of the function, rather than function itself. The definition matching the expected semantics would need to be something like "Pure function is guaranteed to be always called in such a way that it has no side effects." -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
--- Additional Comments From jakub at gcc dot gnu dot org 2005-02-18 15:55
---
Just got hit by this too on subversion.
Distilled testcase is:
typedef __SIZE_TYPE__ size_t;
extern size_t strlen (const char *s);
extern int strncmp (const char *s1, const char *s2, size_t n);
extern void abort (void);
const char *a[16] = { "a", "bc", "de", "fgh" };
int
foo (char *x, const char *y, size_t n)
{
size_t i, j = 0;
for (i = 0; i < n; i++)
{
if (strncmp (x + j, a[i], strlen (a[i])) != 0)
return 2;
j += strlen (a[i]);
if (y)
j += strlen (y);
}
return 0;
}
int
main (void)
{
if (foo ("abcde", (const char *) 0, 3) != 0)
abort ();
return 0;
}
With Zdenek's definition of pure the only pure functions in builtins.def
would be is*/to*, none of the string/memory functions could be. But the
documentation explicitely mentions strlen and memcmp as pure functions.
--
What|Removed |Added
CC||jakub at redhat dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
--- Additional Comments From joseph at codesourcery dot com 2005-02-13 20:38 --- Subject: Re: [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory On Sun, 13 Feb 2005, drow at gcc dot gnu dot org wrote: > I've suggested when talking to someone else about this that they should be > assumed not to trap at the points where they are called. That would allow > calls to them to be removed, although still limit code motion. > > It's a pity there's no rigorous definition of the current meaning of pure. I would say that a pure function is a pure but not necessarily total function of its arguments and those parts of the state of the machine visible to it: you can move or remove calls to pure functions as long as in the abstract machine the function would have been called with the particular machine state with which it does get called. So strlen is pure and "if (s) strlen(s);" is valid and the strlen call can't be moved before the test for NULL. Similarly a const function is a pure but not necessarily total function of the values of its arguments only: so a square root function which doesn't set errno or floating point exception flags could be "const" and a call to it should not be moved before a test that the argument is nonnegative. Where there are optimizations depending on the function being total - not trapping even if you call it with arguments with which the program never calls it in the abstract machine model - perhaps an additional attribute "total" should be added which can be used in conjunction with "const" or "pure". (Though I'm not sure how many real functions are going to be "pure" and "total" but not "const".) -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
--- Additional Comments From ian at airs dot com 2005-02-13 20:27 --- The definition of pure states that such a function may depend upon global variables. I guess you are saying that func_pure_2 is not pure because the global variable a may not be a valid memory address? I disagree. If the programmer declares that func_pure_2 is pure, then the programmer is promising the compiler that the variable a holds a valid memory address. The attribute is not a statement of something which the compiler can deduce for itself; it is a promise by the programmer. Given that promise, the compiler is fully entitled and expected to hoist function calls out of loops which do not modify global variables, etc. If that causes a bug, it is a bug in the code, not in the compiler. I agree that it would be nice if the compiler could do a better job of analyzing whether a function is const/pure, but until then we should trust the programmer. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
--- Additional Comments From drow at gcc dot gnu dot org 2005-02-13 20:13 --- I've suggested when talking to someone else about this that they should be assumed not to trap at the points where they are called. That would allow calls to them to be removed, although still limit code motion. It's a pity there's no rigorous definition of the current meaning of pure. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
--- Additional Comments From rakdver at atrey dot karlin dot mff dot cuni dot cz 2005-02-13 20:11 --- Subject: Re: [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory > That's a pretty useless definition of pure functions - they may read global > memory, but not dereference any pointers which are invalid at any point in > the life of the program? sorry, but allowing pure functions to trap would make them even more useless. For example it would be forbidden to remove calls to them (no dce), possibilities for code motion would be severely limited, etc. Hopefully with interprocedural alias analysis pure specifier will become less needed. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
--- Additional Comments From drow at gcc dot gnu dot org 2005-02-13 20:05 --- That's a pretty useless definition of pure functions - they may read global memory, but not dereference any pointers which are invalid at any point in the life of the program? -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
--- Additional Comments From rakdver at gcc dot gnu dot org 2005-02-13 19:50 --- Function that may trap is not pure; in particular func_pure_2 cannot be considered pure. The remaining testcases are real bugs, however. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
--- Additional Comments From steven at gcc dot gnu dot org 2005-02-12 14:52 --- Zdenek, a tree-ssa-loop-im problem, apparently... -- What|Removed |Added CC||rakdver at gcc dot gnu dot ||org, steven at gcc dot gnu ||dot org http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
--- Additional Comments From steven at gcc dot gnu dot org 2005-02-10 10:58 --- wrong-code, the worst kind we have... -- What|Removed |Added Severity|normal |critical Priority|P2 |P1 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-02-09 07:03 --- Confirmed via Daniel Berline via IRC and Drow via this bug. -- What|Removed |Added Status|UNCONFIRMED |NEW Ever Confirmed||1 Last reconfirmed|-00-00 00:00:00 |2005-02-09 07:03:08 date|| http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-02-08
20:02 ---
(In reply to comment #2)
> Here's another one. This may be a different bug.
Yes that is a different (but related) bug. The problem is now, what is
definition of pure functions (for
that testcase).
Take the following:
int *a;
int *func_pure () __attribute__((pure));
int func_pure_2 () __attribute__((pure));
int *func_pure() { return a; }
int func_pure_2() { return *a;}
int i;
int
func_loop_3 (int arg)
{
int var = 0;
i = 1;
while (arg--)
{
if (func_pure ())
var = func_pure_2 ();
}
return var;
}
void abort (void);
int main()
{
int i;
i = func_loop_3 (10);
if (i)
abort ();
}
Is func_pure_2 really pure and if it is, then we should change it so we can
trap on it.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
--- Additional Comments From drow at gcc dot gnu dot org 2005-02-08 19:36
---
Here's another one. This may be a different bug. Suppose we have two pure
functions, one which checks whether a library is present and one which fetches
some piece of data from the library. Code looks like this:
int
func_loop_3 (int arg)
{
int var = 0;
while (arg--)
{
if (func_pure ())
var = func_pure_2 ();
}
return var;
}
LIM will move _both_ pure calls out of the loop. I think that it is valid
for a pure call to segfault in a condition when it would not normally have
been called; if the implementation of func_pure always returns zero, I don't
think that func_pure_2 should ever be called in the above.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
--- Additional Comments From drow at gcc dot gnu dot org 2005-02-08 19:27
---
Here's another related testcase. If you uncomment the store to global_int,
LIM will move only func_const out of the loop. With them both commented
out, however, the pure call gets moved out of the loop. func_other may
modify global memory, though, so the pure call can not be moved.
int func_pure (void) __attribute__ ((pure));
int func_const (void) __attribute__ ((const));
void func_other (int);
int global_int;
int
func_loop (int arg)
{
while (arg--)
{
// global_int = arg;
func_other (func_pure ());
}
}
int
func_loop_2 (int arg)
{
while (arg--)
{
// global_int = arg;
func_other (func_const ());
}
}
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
-- What|Removed |Added CC||ian at airs dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
[Bug tree-optimization/19828] [4.0 Regression] LIM is pulling out a pure function even though there is something which can modify global memory
-- What|Removed |Added Target Milestone|--- |4.0.0 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19828
