Based on the observation that given the following:
static void my_callback(void)
{
}
void (*callback)(void) = 0 ? my_callback : NULL;
GCC will,
1.) Not emit 'defined but unused' warnings for 'my_callback'
2.) Typecheck my_callback against typeof(*callback)
3.) Eliminate my_callback as dead code (assuming it's unused elsewhere)
this patchset explores where/how this might be used to reduce the amount of
ifdeffed code in device drivers.
Patch 1 in this series introduces two friendly-named helper macros:
assign_if(const_expr, fn): A friendlier form of (const_expr ? fn :
NULL)
assign_if_enabled(conf, fn): Composition of assign_if() and
IS_ENABLED().
In order to show a good target usecase for these macros, patch 2 introduces a
new set of PM-related macros, similar to SET_*_PM_OPS, using
assign_if_enabled() under the hood.
In particular, this is aimed at reducing code like the following:
#ifdef CONFIG_PM_SLEEP
static int foo_suspend(struct device *dev)
{
/* ... */
}
#else
#define foo_suspend NULL
#endif
static const struct dev_pm_ops foo_pm_ops = {
SET_SYSTEM_SLEEP_PM_OPS(foo_suspend, NULL)
};
With the ifdefless:
static int foo_suspend(struct device *dev)
{
/* ... */
}
static const struct dev_pm_ops foo_pm_ops = {
ASSIGN_SYSTEM_SLEEP_PM_OPS(foo_suspend, NULL),
};
For patch 3, an existing consumer of SET_*_PM_OPS(), the MSM USB phy driver[1]
is modified to use the newly introduced ASSIGN_*_PM_OPS() macros to show in a
real case what simplifications could be gained.
For testing that GCC is actually eliminating the PM-related callbacks when it
can, the MSM USB phy driver was built before and after patch 3, in 4 different
configurations, and object sizes compared[2]:
text data bss dec hex filename
11008 488 20 11516 2cfc phy_objects_before/phy-msm-usb.nopm.o
11008 488 20 11516 2cfc phy_objects_after/phy-msm-usb.nopm.o
13528 584 20 14132 3734 phy_objects_before/phy-msm-usb.runtime.o
13528 584 20 14132 3734 phy_objects_after/phy-msm-usb.runtime.o
13828 632 20 14480 3890
phy_objects_before/phy-msm-usb.runtime+sleep.o
13828 632 20 14480 3890
phy_objects_after/phy-msm-usb.runtime+sleep.o
13072 560 20 13652 3554 phy_objects_before/phy-msm-usb.sleep.o
13072 560 20 13652 3554 phy_objects_after/phy-msm-usb.sleep.o
[1]: Chosen because it has recently broken randconfig builds due to improper
#ifdeffery using CONFIG_PM* defines
[2]:
$ ${CROSS_COMPILE}gcc --version
arm-linux-gnueabihf-gcc (crosstool-NG linaro-1.13.1-4.8-2013.07-1 -
Linaro GCC 2013.07) 4.8.2 20130624 (prerelease)
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is
NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
PURPOSE.
Josh Cartwright (3):
typecheck: introduce assign_if() and assign_if_enabled()
PM: define new ASSIGN_*_PM_OPS macros based on assign_if
usb: phy: msm: use ASSIGN_*_PM_OPS variants
drivers/usb/phy/phy-msm-usb.c | 13 +++----------
include/linux/pm.h | 39 +++++++++++++++++++++++++++++++++++++++
include/linux/typecheck.h | 18 ++++++++++++++++++
3 files changed, 60 insertions(+), 10 deletions(-)
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