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

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

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.

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:;