[CodeGen] Replace -max-jump-table-size with -max-jump-table-targets

Authored by evandro on Sep 25 2019, 9:10 AM.


[CodeGen] Replace -max-jump-table-size with -max-jump-table-targets

Modern processors predict the targets of an indirect branch regardless of
the size of any jump table used to glean its target address. Moreover,
branch predictors typically use resources limited by the number of actual
targets that occur at run time.

This patch changes the semantics of the option -max-jump-table-size to limit
the number of different targets instead of the number of entries in a jump
table. Thus, it is now renamed to -max-jump-table-targets.

Before, when -max-jump-table-size was specified, it could happen that
cluster jump tables could have targets used repeatedly, but each one was
counted and typically resulted in tables with the same number of entries.
With this patch, when specifying -max-jump-table-targets, tables may have
different lengths, since the number of unique targets is counted towards the
limit, but the number of unique targets in tables is the same, but for the
last one containing the balance of targets.

Differential revision: https://reviews.llvm.org/D60295

llvm-svn: 372893