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))?
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:;
