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

[Xinliang David Li <davidxl at google dot com>]

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.

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:;