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Re: [PATCH] Add -feliminate-malloc to enable/disable elimination of redundant malloc/free pairs


On Wed, May 9, 2012 at 11:21 AM, Richard Guenther
<richard.guenther@gmail.com> wrote:
> On Wed, May 9, 2012 at 6:32 PM, Xinliang David Li <davidxl@google.com> wrote:
>> On Wed, May 9, 2012 at 1:12 AM, Richard Guenther
>> <richard.guenther@gmail.com> wrote:
>>> On Tue, May 8, 2012 at 6:18 PM, Xinliang David Li <davidxl@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))?
>>>
>>
>> The side effect is the user registered malloc hooks.
>>
>> The pattern 'free(malloc(4)', or loops like
>>
>> for (..)
>> ?{
>> ? ?malloc(..); // result not used
>> ?}
>>
>> itself is not interesting. They only appear in the test code and the
>> problem can be worked around by using -fno-builtin-malloc. The option
>> is to avoid disable this and all similar malloc optimizations (in the
>> future) for malloc implementation with hooks.
>
> What is then interesting? ?The above are all the same as if optimization
> figured out the storage is not used, no?

Compiler may choose to convert malloc call into alloca or coalesce
multiple malloc calls -- but this may not always be the best choice
given that more advanced locality based heap allocation is possible.
Nothing like this exist yet, but the point is that it is convenient to
have a way to keep malloc call while keeping its aliasing property.

David

>
> Richard.
>
>> Thanks,
>>
>> David
>>
>>> Richard.
>>>
>>>> David
>>>>
>>>> On Tue, May 8, 2012 at 7:43 AM, Dehao Chen <dehao@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 ?<dehao@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 ?<dehao@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:;


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