Hi!
As mentioned in the PR, RTL DSE doesn't do much with -fstack-protector*,
because the stack canary test in the epilogue of instrumented functions
is a MEM_VOLATILE_P read out of the crtl->stack_protect_guard ssp canary
slot in the stack frame and either a MEM_VOLATILE_P read of
__stack_chk_guard variable, or corresponding some other location (e.g. TLS
memory on x86).
The canary slot in the stack frame is written in the prologue using
MEM_VOLATILE_P store, so we never consider those to be DSEd and is only read
in the epilogue, so it shouldn't alias any other stores.
Similarly, __stack_chk_guard variable or say the TLS ssp slot or whatever
else is used to hold the random pointer-sized value really shouldn't be
changed in -fstack-protector* instrumented functions, as that would mean
they remembered one value in the prologue and would fail comparison in the
epilogue if it changed in between. So, I believe we can safely ignore the
whole stack_pointer_test instruction in RTL DSE.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2019-01-10 Jakub Jelinek <ja...@redhat.com>
PR rtl-optimization/88796
* emit-rtl.h (struct rtl_data): Add stack_protect_guard_decl field.
* cfgexpand.c (stack_protect_prologue): Initialize
crtl->stack_protect_guard_decl.
* function.c (stack_protect_epilogue): Use it instead of calling
targetm.stack_protect_guard again.
* dse.c (check_mem_read_rtx): Ignore MEM_VOLATILE_P reads from
MEMs with MEM_EXPR equal to crtl->stack_protect_guard or
crtl->stack_protect_guard_decl.
* config/i386/i386.c (ix86_stack_protect_guard): Set TREE_THIS_VOLATILE
on the returned MEM_EXPR.
* gcc.target/i386/pr88796.c: New test.
--- gcc/emit-rtl.h.jj 2019-01-10 11:43:14.390377646 +0100
+++ gcc/emit-rtl.h 2019-01-10 21:38:38.682055891 +0100
@@ -87,6 +87,10 @@ struct GTY(()) rtl_data {
Used for detecting stack clobbers. */
tree stack_protect_guard;
+ /* The __stack_chk_guard variable or expression holding the stack
+ protector canary value. */
+ tree stack_protect_guard_decl;
+
/* List (chain of INSN_LIST) of labels heading the current handlers for
nonlocal gotos. */
rtx_insn_list *x_nonlocal_goto_handler_labels;
--- gcc/cfgexpand.c.jj 2019-01-07 09:50:26.774650762 +0100
+++ gcc/cfgexpand.c 2019-01-10 21:40:08.714589919 +0100
@@ -6219,6 +6219,7 @@ stack_protect_prologue (void)
tree guard_decl = targetm.stack_protect_guard ();
rtx x, y;
+ crtl->stack_protect_guard_decl = guard_decl;
x = expand_normal (crtl->stack_protect_guard);
if (targetm.have_stack_protect_combined_set () && guard_decl)
--- gcc/function.c.jj 2019-01-10 16:43:54.802481070 +0100
+++ gcc/function.c 2019-01-10 21:40:49.326928642 +0100
@@ -4902,7 +4902,7 @@ init_function_start (tree subr)
void
stack_protect_epilogue (void)
{
- tree guard_decl = targetm.stack_protect_guard ();
+ tree guard_decl = crtl->stack_protect_guard_decl;
rtx_code_label *label = gen_label_rtx ();
rtx x, y;
rtx_insn *seq = NULL;
--- gcc/dse.c.jj 2019-01-10 11:43:12.345411240 +0100
+++ gcc/dse.c 2019-01-10 21:48:07.224799798 +0100
@@ -2072,8 +2072,29 @@ check_mem_read_rtx (rtx *loc, bb_info_t
insn_info = bb_info->last_insn;
if ((MEM_ALIAS_SET (mem) == ALIAS_SET_MEMORY_BARRIER)
- || (MEM_VOLATILE_P (mem)))
+ || MEM_VOLATILE_P (mem))
{
+ if (crtl->stack_protect_guard
+ && (MEM_EXPR (mem) == crtl->stack_protect_guard
+ || (crtl->stack_protect_guard_decl
+ && MEM_EXPR (mem) == crtl->stack_protect_guard_decl))
+ && MEM_VOLATILE_P (mem))
+ {
+ /* This is either the stack protector canary on the stack,
+ which ought to be written by a MEM_VOLATILE_P store and
+ thus shouldn't be deleted and is read at the very end of
+ function, but shouldn't conflict with any other store.
+ Or it is __stack_chk_guard variable or TLS or whatever else
+ MEM holding the canary value, which really shouldn't be
+ ever modified in -fstack-protector* protected functions,
+ otherwise the prologue store wouldn't match the epilogue
+ check. */
+ if (dump_file && (dump_flags & TDF_DETAILS))
+ fprintf (dump_file, " stack protector canary read ignored.\n");
+ insn_info->cannot_delete = true;
+ return;
+ }
+
if (dump_file && (dump_flags & TDF_DETAILS))
fprintf (dump_file, " adding wild read, volatile or barrier.\n");
add_wild_read (bb_info);
--- gcc/config/i386/i386.c.jj 2019-01-10 11:43:17.534325998 +0100
+++ gcc/config/i386/i386.c 2019-01-10 21:35:39.588972002 +0100
@@ -45093,6 +45093,7 @@ ix86_stack_protect_guard (void)
t = build_int_cst (asptrtype, ix86_stack_protector_guard_offset);
t = build2 (MEM_REF, asptrtype, t,
build_int_cst (asptrtype, 0));
+ TREE_THIS_VOLATILE (t) = 1;
}
return t;
--- gcc/testsuite/gcc.target/i386/pr88796.c.jj 2019-01-10 21:58:48.878354306
+0100
+++ gcc/testsuite/gcc.target/i386/pr88796.c 2019-01-10 21:58:42.468458654
+0100
@@ -0,0 +1,8 @@
+/* PR rtl-optimization/88796 */
+/* { dg-do compile { target { ! ia32 } } } */
+/* { dg-options "-O2 -fstack-protector-strong" } */
+/* { dg-require-effective-target fstack_protector } */
+
+#include "pr87370.c"
+
+/* { dg-final { scan-assembler-not "xmm" } } */
Jakub
