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:;
