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 <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:;
