On Wed, May 9, 2012 at 11:48 AM, Dehao Chen <de...@google.com> wrote:
> On Wed, May 9, 2012 at 5:22 PM, Richard Guenther
> <richard.guent...@gmail.com> wrote:
>> On Wed, May 9, 2012 at 10:38 AM, Dehao Chen <de...@google.com> wrote:
>>> On Wed, May 9, 2012 at 4:12 PM, Richard Guenther
>>> <richard.guent...@gmail.com> wrote:
>>>> On Tue, May 8, 2012 at 6:18 PM, Xinliang David Li <davi...@google.com>
>>>> wrote:
>>>>> To be clear, this flag is for malloc implementation (such as tcmalloc)
>>>>> with side effect unknown to the compiler. Using -fno-builtin-xxx is
>>>>> too conservative for that purpose.
>>>>
>>>> I don't think that flys. Btw, the patch also guards alloca - alloca is
>>>> purely
>>>> GCC internal.
>>>>
>>>> What's the "unknown side-effects" that are also important to preserve
>>>> for free(malloc(4))?
>>>
>>> Malloc implementation may record some info to a global structure, and
>>> a program may use this free(malloc()) pair to simulate the real runs
>>> to get some data, such as peak memory requirement.
>>
>> So why not use an alternate interface into this special allocator for this
>> purpose?
>
> There can be the following scenario:
>
> We want to add a module to an existing app. Before implementing the
> module, we want to collect some statistics on real runs. In this
> scenario, we need:
>
> * No change to the legacy code
> * Optimized build for the simulation run
> * Provide accurate statistical info
>
> We want to collect data for both new module and the legacy code
> without changing the later, thus we cannot use a new malloc/free
> interface.
So put in a optimization barrier then. Like
free (({ void * x = malloc (4); __asm ("" : "+m" (x)); __x; });
Btw, why can't you simply build the new module (which doesn't something
real anyway, just fake stuff) without optimization?
> Thanks,
> Dehao
>
>>
>>> Dehao
>>>
>>>>
>>>> Richard.
>>>>
>>>>> David
>>>>>
>>>>> On Tue, May 8, 2012 at 7:43 AM, Dehao Chen <de...@google.com> wrote:
>>>>>> Hello,
>>>>>>
>>>>>> This patch adds a flag to guard the optimization that optimize the
>>>>>> following code away:
>>>>>>
>>>>>> free (malloc (4));
>>>>>>
>>>>>> In some cases, we'd like this type of malloc/free pairs to remain in
>>>>>> the optimized code.
>>>>>>
>>>>>> Tested with bootstrap, and no regression in the gcc testsuite.
>>>>>>
>>>>>> Is it ok for mainline?
>>>>>>
>>>>>> Thanks,
>>>>>> Dehao
>>>>>>
>>>>>> gcc/ChangeLog
>>>>>> 2012-05-08 Dehao Chen <de...@google.com>
>>>>>>
>>>>>> * common.opt (feliminate-malloc): New.
>>>>>> * doc/invoke.texi: Document it.
>>>>>> * tree-ssa-dce.c (mark_stmt_if_obviously_necessary): Honor it.
>>>>>>
>>>>>> gcc/testsuite/ChangeLog
>>>>>> 2012-05-08 Dehao Chen <de...@google.com>
>>>>>>
>>>>>> * gcc.dg/free-malloc.c: Check if -fno-eliminate-malloc is working
>>>>>> as expected.
>>>>>>
>>>>>> Index: gcc/doc/invoke.texi
>>>>>> ===================================================================
>>>>>> --- gcc/doc/invoke.texi (revision 187277)
>>>>>> +++ gcc/doc/invoke.texi (working copy)
>>>>>> @@ -360,7 +360,8 @@
>>>>>> -fcx-limited-range @gol
>>>>>> -fdata-sections -fdce -fdelayed-branch @gol
>>>>>> -fdelete-null-pointer-checks -fdevirtualize -fdse @gol
>>>>>> --fearly-inlining -fipa-sra -fexpensive-optimizations -ffat-lto-objects
>>>>>> @gol
>>>>>> +-fearly-inlining -feliminate-malloc -fipa-sra -fexpensive-optimizations
>>>>>> @gol
>>>>>> +-ffat-lto-objects @gol
>>>>>> -ffast-math -ffinite-math-only -ffloat-store
>>>>>> -fexcess-precision=@var{style} @gol
>>>>>> -fforward-propagate -ffp-contract=@var{style} -ffunction-sections @gol
>>>>>> -fgcse -fgcse-after-reload -fgcse-las -fgcse-lm -fgraphite-identity @gol
>>>>>> @@ -6238,6 +6239,7 @@
>>>>>> -fdefer-pop @gol
>>>>>> -fdelayed-branch @gol
>>>>>> -fdse @gol
>>>>>> +-feliminate-malloc @gol
>>>>>> -fguess-branch-probability @gol
>>>>>> -fif-conversion2 @gol
>>>>>> -fif-conversion @gol
>>>>>> @@ -6762,6 +6764,11 @@
>>>>>> Perform dead store elimination (DSE) on RTL@.
>>>>>> Enabled by default at @option{-O} and higher.
>>>>>>
>>>>>> +@item -feliminate-malloc
>>>>>> +@opindex feliminate-malloc
>>>>>> +Eliminate unnecessary malloc/free pairs.
>>>>>> +Enabled by default at @option{-O} and higher.
>>>>>> +
>>>>>> @item -fif-conversion
>>>>>> @opindex fif-conversion
>>>>>> Attempt to transform conditional jumps into branch-less equivalents.
>>>>>> This
>>>>>> Index: gcc/testsuite/gcc.dg/free-malloc.c
>>>>>> ===================================================================
>>>>>> --- gcc/testsuite/gcc.dg/free-malloc.c (revision 0)
>>>>>> +++ gcc/testsuite/gcc.dg/free-malloc.c (revision 0)
>>>>>> @@ -0,0 +1,12 @@
>>>>>> +/* { dg-do compile } */
>>>>>> +/* { dg-options "-O2 -fno-eliminate-malloc" } */
>>>>>> +/* { dg-final { scan-assembler-times "malloc" 2} } */
>>>>>> +/* { dg-final { scan-assembler-times "free" 2} } */
>>>>>> +
>>>>>> +extern void * malloc (unsigned long);
>>>>>> +extern void free (void *);
>>>>>> +
>>>>>> +void test ()
>>>>>> +{
>>>>>> + free (malloc (10));
>>>>>> +}
>>>>>> Index: gcc/common.opt
>>>>>> ===================================================================
>>>>>> --- gcc/common.opt (revision 187277)
>>>>>> +++ gcc/common.opt (working copy)
>>>>>> @@ -1474,6 +1474,10 @@
>>>>>> Common Var(flag_dce) Init(1) Optimization
>>>>>> Use the RTL dead code elimination pass
>>>>>>
>>>>>> +feliminate-malloc
>>>>>> +Common Var(flag_eliminate_malloc) Init(1) Optimization
>>>>>> +Eliminate unnecessary malloc/free pairs
>>>>>> +
>>>>>> fdse
>>>>>> Common Var(flag_dse) Init(1) Optimization
>>>>>> Use the RTL dead store elimination pass
>>>>>> Index: gcc/tree-ssa-dce.c
>>>>>> ===================================================================
>>>>>> --- gcc/tree-ssa-dce.c (revision 187277)
>>>>>> +++ gcc/tree-ssa-dce.c (working copy)
>>>>>> @@ -309,6 +309,8 @@
>>>>>> case BUILT_IN_CALLOC:
>>>>>> case BUILT_IN_ALLOCA:
>>>>>> case BUILT_IN_ALLOCA_WITH_ALIGN:
>>>>>> + if (!flag_eliminate_malloc)
>>>>>> + mark_stmt_necessary (stmt, true);
>>>>>> return;
>>>>>>
>>>>>> default:;
