[llvm-commits] [Review request] [PR8714] Jump threading of indirectbr blocks (and more)

[Frits van Bommel]

The first four attached patches implement jump threading of
'indirectbr' instructions when the address being jumped to is a known
blockaddress in a predecessor (or just a constant block address), in
these stages:
0) Remove trailing whitespace.
1) Refactor jump threading. Should have no functional change other
than the order of two transformations that are mutually-exclusive[1]
and the exact formatting of debug output. Internally, it now stores
the ConstantInt*s as Constant*s, and actual undef values instead of
nulls. Splitting this out hopefully makes the actual patch much
simpler to review.
2) Add a test case for (3).
3) Actually implement jump threading of 'indirectbr' by keeping track
of whether we're looking for ConstantInt*s or BlockAddress*s.

[1]: I had to switch them because that was the only place where it
used to test for ConstantInt without also testing for UndefinedValue,
and I wanted to factor out the (ConstantInt || UndefValue) test so I
could add BlockAddress later.

By the way, I wasn't sure what "bonus" to give for threading over an
indirectbr. I figured a bit more than the one for threading over a
switch, but I could be completely off base here. Anyone?


While working on this I noticed that "opt -O3" turned the phi in my
original testcase into a select and didn't further optimize it, so
that the indirectbr wasn't eliminated. To fix this, patch #5 teaches
SimplifyCFG (which created the select) to simplify (indirectbr (select
cond, blockaddress(@fn, BlockA), blockaddress(@fn, BlockB))) to (br
cond, BlockA, BlockB). Patch #4 is a test for this.


Comments?
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