On 03/25/11 10:39, Richard Guenther wrote:
On Fri, Mar 25, 2011 at 4:33 PM, Jakub Jelinek<ja...@redhat.com> wrote:
On Fri, Mar 25, 2011 at 09:30:22AM -0600, Jeff Law wrote:
I'm not going to chime in on this specific problem; however, it is worth
noting that many of the issues raised by the C++0x memory model also
affect the linux kernel.
But what they are seeing are certainly store data races, not load races,
because no hw they care about (or no hw at all?) detects the latter.
Having options to avoid store data races is useful not just for C++0x memory
model compliance and Linux kernel, but e.g. for OpenMP too.
And we have in the past removed code that created store data races
anyway. There is nothing new here.
As stated multiple times in the past things get "interesting" when you
look at non-x86 hardware.
Agreed on all counts, with everyone :). I will desist from trying to
disable speculative loads and load data races which affect no one.
Below is a patch I am committing to remove all tests checking for load
races.
As Richi has mentioned, there are some interesting store data races on
non-x86 hardware. I will be looking into those, particularly non
contiguous bitfield issues on ppc, s390, and possibly alpha: stay tuned.
Thanks for the comments.
* gcc.dg/memmodel/d2.c: Remove.
* gcc.dg/memmodel/d3.c: Remove.
* gcc.dg/memmodel/global-hoist.c: Remove.
Index: gcc.dg/memmodel/global-hoist.c
===================================================================
--- gcc.dg/memmodel/global-hoist.c (revision 170745)
+++ gcc.dg/memmodel/global-hoist.c (working copy)
@@ -1,73 +0,0 @@
-/* { dg-do link } */
-/* { dg-options "-O2 --param allow-load-data-races=0" } */
-/* { dg-final { memmodel-gdb-test } } */
-
-/* Verify that a load of a global is not hoisted out of a loop. This
- can introduce a data race, and is dissallowed if
- --param allow-load-data-races is 0. */
-
-#include <stdio.h>
-#include <stdlib.h>
-#include "memmodel.h"
-
-int global = 0;
-int sum[5];
-
-
-/* The other thread in the system increments the value of global by
- one each time it is executed. By accessing 'global' in a loop, we can
- check if the load has been hoisted out of the loop by seeing if
- iterations of the loop get the same or different values of
- global. */
-void
-memmodel_other_threads()
-{
- global++;
-}
-
-/* Nothing to verify at each step. */
-int
-memmodel_step_verify()
-{
- return 0;
-}
-
-/* Every element of the array should have a different value of global if the
- * load is left in the loop like it is supposed to. */
-int
-memmodel_final_verify()
-{
- int ret = 0;
- int x, y;
-
- for (x = 0; x < 4; x++)
- for (y = x + 1; y < 5; y++)
- {
- if (sum[x] == sum[y])
- {
- printf("FAIL: sum[%d] and sum[%d] have the same value : %d\n",
- x, y, sum[x]);
- ret = 1;
- }
- }
- return ret;
-}
-
-/* 'global' is bumped by "the other thread" every insn. Test that all
- elements of 'sum' are different, otherwise load of 'global' has
- been hoisted. */
-void
-test()
-{
- int x;
-
- for (x=0; x< 5; x++)
- sum[x] = global;
-}
-
-int
-main()
-{
- test();
- memmodel_done();
-}
Index: gcc.dg/memmodel/d2.c
===================================================================
--- gcc.dg/memmodel/d2.c (revision 170745)
+++ gcc.dg/memmodel/d2.c (working copy)
@@ -1,60 +0,0 @@
-/* { dg-do link } */
-/* { dg-options "-O2 --param allow-load-data-races=0" } */
-/* { dg-final { memmodel-gdb-test } } */
-
-#include <stdio.h>
-#include "memmodel.h"
-
-/* This is a variation on global-hoist.c where instead of a loop, we
- store global into different elements of an array in straightline
- code with an if condition. This will catch cases where commoning
- should be disabled when --param allow-load-data-races is 0. */
-
-/* Test the FALSE path in test. */
-
-int global = 0;
-int sum[4] = { 0, 0, 0, 0 };
-
-void memmodel_other_threads()
-{
- global++;
-}
-
-int memmodel_step_verify()
-{
- return 0;
-}
-
-int memmodel_final_verify()
-{
- int ret = 0;
- int x, y;
- for (x = 0; x < 3; x++)
- for (y = x + 1; y < 4; y++)
- {
- if (sum[x] == sum[y])
- {
- printf("FAIL: sum[%d] and sum[%d] have the same value : %d\n",
- x, y, sum[x]);
- ret = 1;
- }
- }
-}
-
-/* Since global is always being increased by the 'other' thread, all
- elements of sum should be different. (no cse) */
-int test (int y)
-{
- sum[0] = global;
- if (y)
- sum[1] = global;
- else
- sum[2] = global;
- sum[3] = global;
-}
-
-int main()
-{
- test(0);
- memmodel_done();
-}
Index: gcc.dg/memmodel/d3.c
===================================================================
--- gcc.dg/memmodel/d3.c (revision 170745)
+++ gcc.dg/memmodel/d3.c (working copy)
@@ -1,61 +0,0 @@
-/* { dg-do link } */
-/* { dg-options "-O2 --param allow-load-data-races=0" } */
-/* { dg-final { memmodel-gdb-test } } */
-
-#include <stdio.h>
-#include "memmodel.h"
-
-/* A variation on global-hoist.c where instead of a loop, we store
- global into different elements of an array in straightline code
- with an if condition. This will catch cases where commoning should
- be disabled when --param allow-load-data-races is 0. */
-
-/* Test the TRUE path in test. */
-
-int global = 0;
-int sum[4] = { 0, 0, 0, 0 };
-
-void memmodel_other_threads()
-{
- global++;
-}
-
-int memmodel_step_verify()
-{
- return 0;
-}
-
-int memmodel_final_verify()
-{
- int ret = 0;
- int x, y;
- for (x = 0; x < 3; x++)
- for (y = x + 1; y < 4; y++)
- {
- if (sum[x] == sum[y])
- {
- printf("FAIL: sum[%d] and sum[%d] have the same value : %d\n",
- x, y, sum[x]);
- ret = 1;
- }
- }
-}
-
-/* Since global is always being increased by the 'other' thread, all
- elements of sum should be different. (No CSE) */
-int test(int y)
-{
- sum[0] = global;
- if (y)
- sum[1] = global;
- else
- sum[2] = global;
- sum[3] = global;
-}
-
-
-int main()
-{
- test(1);
- memmodel_done();
-}
