On 3/7/19 11:27 AM, Uros Bizjak wrote:
> On Thu, Mar 7, 2019 at 10:50 AM Martin Liška <mli...@suse.cz> wrote:
>>
>> On 3/7/19 9:54 AM, Uros Bizjak wrote:
>>> On Thu, Mar 7, 2019 at 9:45 AM Martin Liška <mli...@suse.cz> wrote:
>>>>
>>>> Hi.
>>>>
>>>> Thanks to Intel guys, we've done some re-measurement in PR86952
>>>> about usage of jump tables when retpolines are used.
>>>> Numbers prove that disabling of JT should be the best for now.
>>>>
>>>> Patch can bootstrap on x86_64-linux-gnu and survives regression tests.
>>>>
>>>> Ready to be installed?
>>>> Thanks,
>>>> Martin
>>>
>>> Please add a comment above your change.
>>
>> Sure, should be improved.
>
> Eh, we didn't understand each other... Please add comment here:
Ah, this place :)
>
> + if (ix86_indirect_branch != indirect_branch_keep
> + && !opts_set->x_flag_jump_tables)
> + opts->x_flag_jump_tables = 0;
>
> so in future, it will still be documented why this part of the code is needed.
Sure, updated.
Martin
>
> Uros.
>
>> Martin
>>
>>>
>>> Uros.
>>>
>>>>
>>>> gcc/ChangeLog:
>>>>
>>>> 2019-03-06 Martin Liska <mli...@suse.cz>
>>>>
>>>> PR target/86952
>>>> * config/i386/i386.c (ix86_option_override_internal): Disable
>>>> jump tables when retpolines are used.
>>>>
>>>> gcc/testsuite/ChangeLog:
>>>>
>>>> 2019-03-06 Martin Liska <mli...@suse.cz>
>>>>
>>>> PR target/86952
>>>> * gcc.target/i386/pr86952.c: New test.
>>>> * gcc.target/i386/indirect-thunk-7.c: Use jump tables to match
>>>> scanned pattern.
>>>> * gcc.target/i386/indirect-thunk-inline-7.c: Likewise.
>>>> ---
>>>> gcc/config/i386/i386.c | 4 ++++
>>>> .../gcc.target/i386/indirect-thunk-7.c | 2 +-
>>>> .../gcc.target/i386/indirect-thunk-inline-7.c | 2 +-
>>>> gcc/testsuite/gcc.target/i386/pr86952.c | 23 +++++++++++++++++++
>>>> 4 files changed, 29 insertions(+), 2 deletions(-)
>>>> create mode 100644 gcc/testsuite/gcc.target/i386/pr86952.c
>>>>
>>>>
>>
>From f78914272ba7a9fd19db65876a4d4cab576ddf0f Mon Sep 17 00:00:00 2001
From: marxin <mli...@suse.cz>
Date: Wed, 6 Mar 2019 13:05:50 +0100
Subject: [PATCH] x86: Disable jump tables when retpolines are used (PR
target/86952).
Jump tables are implement on x86_64 with:
jmp *.L4(,%rdi,8)
where L4 contains list of labels where to jump. When using retpolines,
the instruction is replaced with:
movq .L4(,%rdi,8), %rax
jmp *%rax
which bypasses/confuses indirect branch predictor and it's slow. In that
case, a decision tree based on if condition is faster.
gcc/ChangeLog:
2019-03-06 Martin Liska <mli...@suse.cz>
PR target/86952
* config/i386/i386.c (ix86_option_override_internal): Disable
jump tables when retpolines are used.
gcc/testsuite/ChangeLog:
2019-03-06 Martin Liska <mli...@suse.cz>
PR target/86952
* gcc.target/i386/pr86952.c: New test.
* gcc.target/i386/indirect-thunk-7.c: Use jump tables to match
scanned pattern.
* gcc.target/i386/indirect-thunk-inline-7.c: Likewise.
---
gcc/config/i386/i386.c | 6 +++++
.../gcc.target/i386/indirect-thunk-7.c | 2 +-
.../gcc.target/i386/indirect-thunk-inline-7.c | 2 +-
gcc/testsuite/gcc.target/i386/pr86952.c | 23 +++++++++++++++++++
4 files changed, 31 insertions(+), 2 deletions(-)
create mode 100644 gcc/testsuite/gcc.target/i386/pr86952.c
diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index c8f9957163b..71e5cfd2897 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -4894,6 +4894,12 @@ ix86_option_override_internal (bool main_args_p,
opts->x_param_values,
opts_set->x_param_values);
+ /* PR86952: jump table usage with retpolines is slow.
+ The PR provides some numbers about the slowness. */
+ if (ix86_indirect_branch != indirect_branch_keep
+ && !opts_set->x_flag_jump_tables)
+ opts->x_flag_jump_tables = 0;
+
return true;
}
diff --git a/gcc/testsuite/gcc.target/i386/indirect-thunk-7.c b/gcc/testsuite/gcc.target/i386/indirect-thunk-7.c
index 3c72036dbaf..53868f46558 100644
--- a/gcc/testsuite/gcc.target/i386/indirect-thunk-7.c
+++ b/gcc/testsuite/gcc.target/i386/indirect-thunk-7.c
@@ -1,5 +1,5 @@
/* { dg-do compile } */
-/* { dg-options "-O2 -mno-indirect-branch-register -mfunction-return=keep -mindirect-branch=thunk -fno-pic" } */
+/* { dg-options "-O2 -mno-indirect-branch-register -mfunction-return=keep -mindirect-branch=thunk -fno-pic -fjump-tables" } */
void func0 (void);
void func1 (void);
diff --git a/gcc/testsuite/gcc.target/i386/indirect-thunk-inline-7.c b/gcc/testsuite/gcc.target/i386/indirect-thunk-inline-7.c
index ea009245a58..e6f064959a1 100644
--- a/gcc/testsuite/gcc.target/i386/indirect-thunk-inline-7.c
+++ b/gcc/testsuite/gcc.target/i386/indirect-thunk-inline-7.c
@@ -1,5 +1,5 @@
/* { dg-do compile } */
-/* { dg-options "-O2 -mno-indirect-branch-register -mfunction-return=keep -mindirect-branch=thunk-inline -fno-pic" } */
+/* { dg-options "-O2 -mno-indirect-branch-register -mfunction-return=keep -mindirect-branch=thunk-inline -fno-pic -fjump-tables" } */
void func0 (void);
void func1 (void);
diff --git a/gcc/testsuite/gcc.target/i386/pr86952.c b/gcc/testsuite/gcc.target/i386/pr86952.c
new file mode 100644
index 00000000000..3ff3e354878
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr86952.c
@@ -0,0 +1,23 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -mindirect-branch=thunk -fdump-tree-switchlower1" } */
+
+int global;
+
+int
+foo (int x)
+{
+ switch (x & 7)
+ {
+ case 0: ; return 1722;
+ case 1: global += 1; return 1060;
+ case 2: ; return 1990;
+ case 3: ; return 1242;
+ case 4: ; return 1466;
+ case 5: ; return 894;
+ case 6: ; return 570;
+ case 7: ; return 572;
+ default: return 0;
+ }
+}
+
+/* { dg-final { scan-tree-dump ";; GIMPLE switch case clusters: 1 2 3 4 5 6 7" "switchlower1" } } */
--
2.21.0
