On 18/07/2023 22:44, Nathan Chancellor wrote:
A proposed update to clang's -Wconstant-logical-operand to warn when the
left hand side is a constant shows the following instance in
nsecs_to_jiffies_timeout() when NSEC_PER_SEC is not a multiple of HZ,
such as CONFIG_HZ=300:
drivers/gpu/drm/i915/gem/i915_gem_wait.c:189:24: warning: use of logical '&&' with constant operand [-Wconstant-logical-operand]
189 | if (NSEC_PER_SEC % HZ &&
| ~~~~~~~~~~~~~~~~~ ^
drivers/gpu/drm/i915/gem/i915_gem_wait.c:189:24: note: use '&' for a bitwise operation
189 | if (NSEC_PER_SEC % HZ &&
| ^~
| &
drivers/gpu/drm/i915/gem/i915_gem_wait.c:189:24: note: remove constant to silence this warning
1 warning generated.
Turn this into an explicit comparison against zero to make the
expression a boolean to make it clear this should be a logical check,
not a bitwise one.
So -Wconstant-logical-operand only triggers when it is a constant but
not zero constant? Why does that make sense is not a kludge to avoid too
much noise?
Personally, it all feels a bit over the top as a warning, since code in
both cases should optimise away. And we may end up papering over it if
it becomes a default.
Then again this patch IMO does make the code more readable, so I am
happy to take this one via our tree. Or either give ack to bring it in
via drm-misc-next:
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Let me know which route works best.
Regards,
Tvrtko
Link: https://reviews.llvm.org/D142609
Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
---
drivers/gpu/drm/i915/gem/i915_gem_wait.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/i915/gem/i915_gem_wait.c b/drivers/gpu/drm/i915/gem/i915_gem_wait.c
index 4a33ad2d122b..d4b918fb11ce 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_wait.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_wait.c
@@ -186,7 +186,7 @@ i915_gem_object_wait(struct drm_i915_gem_object *obj,
static inline unsigned long nsecs_to_jiffies_timeout(const u64 n)
{
/* nsecs_to_jiffies64() does not guard against overflow */
- if (NSEC_PER_SEC % HZ &&
+ if ((NSEC_PER_SEC % HZ) != 0 &&
div_u64(n, NSEC_PER_SEC) >= MAX_JIFFY_OFFSET / HZ)
return MAX_JIFFY_OFFSET;
