On 04/30/2019 12:42 PM, David Laight wrote: > From: Josh Elsasser >> Sent: 29 April 2019 21:02 >> On Apr 29, 2019, at 12:16 PM, Jeff Kirsher <jeffrey.t.kirs...@intel.com> >> wrote: >> >>> From: Björn Töpel <bjorn.to...@intel.com> >>> >>> 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).
Nope, not really, -fno-jump-tables support predates the latter, and both are supported for gcc versions the kernel cares about.
