From: Josh Elsasser
> Sent: 29 April 2019 21:02
> On Apr 29, 2019, at 12:16 PM, Jeff Kirsher <[email protected]>
> wrote:
>
> > From: Björn Töpel <[email protected]>
> >
> > GCC will generate jump tables for switch-statements with more than 5
> > case statements. An entry into the jump table is an indirect call,
> > which means that for CONFIG_RETPOLINE builds, this is rather
> > expensive.
> >
> > This commit replaces the switch-statement that acts on the XDP program
> > result with an if-clause.
>
> Apologies for the noise, but is this patch still required after the
> recent threshold bump[0] and later removal[1] of switch-case jump
> table generation when building with CONFIG_RETPOLINE?
>
> [0]: https://lore.kernel.org/patchwork/patch/1044863/
> [1]: https://lore.kernel.org/patchwork/patch/1054472/
>
> If nothing else the commit message no longer seems accurate.
Looking at those two patches, the second one seems wrong:
# Additionally, avoid generating expensive indirect jumps which
# are subject to retpolines for small number of switch cases.
# clang turns off jump table generation by default when under
- # retpoline builds, however, gcc does not for x86.
- KBUILD_CFLAGS += $(call cc-option,--param=case-values-threshold=20)
+ # retpoline builds, however, gcc does not for x86. This has
+ # only been fixed starting from gcc stable version 8.4.0 and
+ # onwards, but not for older ones. See gcc bug #86952.
+ ifndef CONFIG_CC_IS_CLANG
+ KBUILD_CFLAGS += $(call cc-option,-fno-jump-tables)
+ endif
If -fno-jump-tables isn't supported then --param=case-values-threshold=20
needs to be set (if supported).
David
-
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