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

[Dehao Chen]

On Wed, May 9, 2012 at 4:12 PM, 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))?

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.

Dehao

>
> 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:;