2021-07-01 01:03:30

by Gustavo A. R. Silva

Subject: [PATCH] scsi: aic94xx: Fix fall-through warning for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a
warning by explicitly adding a fallthrough; statement.

Notice that this seems to be a Duff device for performance[1]. So,
although the code looks a bit _funny_, I didn't want to refactor
or modify it beyond merely adding a fallthrough marking, which
might be the least disruptive way to fix this issue.

[1] https://www.drdobbs.com/a-reusable-duff-device/184406208

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <[email protected]>
---
JFYI: We had thousands of these sorts of warnings and now we are down
to just 7 in linux-next(20210630). This is one of those last
remaining warnings. :)

drivers/scsi/aic94xx/aic94xx_sds.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/aic94xx/aic94xx_sds.c b/drivers/scsi/aic94xx/aic94xx_sds.c
index 297a66770260..46815e65f7a4 100644
--- a/drivers/scsi/aic94xx/aic94xx_sds.c
+++ b/drivers/scsi/aic94xx/aic94xx_sds.c
@@ -718,10 +718,12 @@ static void *asd_find_ll_by_id(void * const start, const u8 id0, const u8 id1)
do {
switch (id1) {
default:
- if (el->id1 == id1)
+ if (el->id1 == id1) {
+ fallthrough;
case 0xFF:
if (el->id0 == id0)
return el;
+ }
}
el = start + le16_to_cpu(el->next);
} while (el != start);
--
2.27.0

2021-07-01 16:44:12

by Joe Perches

Subject: Re: [PATCH] scsi: aic94xx: Fix fall-through warning for Clang

On Wed, 2021-06-30 at 20:04 -0500, Gustavo A. R. Silva wrote:
> In preparation to enable -Wimplicit-fallthrough for Clang, fix a
> warning by explicitly adding a fallthrough; statement.
>
> Notice that this seems to be a Duff device for performance[1]. So,
> although the code looks a bit _funny_, I didn't want to refactor
> or modify it beyond merely adding a fallthrough marking, which
> might be the least disruptive way to fix this issue.

If you read the surrounding calling code, it appears it's
not performant.

* @start has to be the _base_ element start, since the
* linked list entries's offset is from this pointer.
* Some linked list entries use only the first id, in which case
* you can pass 0xFF for the second.

> diff --git a/drivers/scsi/aic94xx/aic94xx_sds.c b/drivers/scsi/aic94xx/aic94xx_sds.c
[]
> @@ -718,10 +718,12 @@ static void *asd_find_ll_by_id(void * const start, const u8 id0, const u8 id1)
> ? do {
> ? switch (id1) {
> ? default:
> - if (el->id1 == id1)
> + if (el->id1 == id1) {
> + fallthrough;
> ? case 0xFF:
> ? if (el->id0 == id0)
> ? return el;
> + }
> ? }
> ? el = start + le16_to_cpu(el->next);
> ? } while (el != start);

And this is still horrible code to read.

I think the below is rather more sensible and it doesn't change
the defconfig with aic94xx object size.

$ size drivers/scsi/aic94xx/aic94xx_sds.o*
text data bss dec hex filename
10010 192 0 10202 27da drivers/scsi/aic94xx/aic94xx_sds.o.new
10010 192 0 10202 27da drivers/scsi/aic94xx/aic94xx_sds.o.old
---
drivers/scsi/aic94xx/aic94xx_sds.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/scsi/aic94xx/aic94xx_sds.c b/drivers/scsi/aic94xx/aic94xx_sds.c
index 297a66770260c..069278f6d6cea 100644
--- a/drivers/scsi/aic94xx/aic94xx_sds.c
+++ b/drivers/scsi/aic94xx/aic94xx_sds.c
@@ -717,11 +717,14 @@ static void *asd_find_ll_by_id(void * const start, const u8 id0, const u8 id1)

do {
switch (id1) {
- default:
- if (el->id1 == id1)
case 0xFF:
- if (el->id0 == id0)
- return el;
+ if (el->id0 == id0)
+ return el;
+ break;
+ default:
+ if (el->id0 == id0 && el->id1 == id1)
+ return el;
+ break;
}
el = start + le16_to_cpu(el->next);
} while (el != start);