diff options
author | Hans Wennborg <hans@hanshq.net> | 2014-01-15 05:00:27 +0000 |
---|---|---|
committer | Hans Wennborg <hans@hanshq.net> | 2014-01-15 05:00:27 +0000 |
commit | 89fa06ba0f54cc90dbdce4f6a91d09f30f033e0a (patch) | |
tree | fd021a5cd5388589f0382cc26741f45cb525a286 /lib/Transforms/Utils/SimplifyCFG.cpp | |
parent | 27a41c154721b38f0192d9bd2e1d90e43f5083e4 (diff) | |
download | llvm-89fa06ba0f54cc90dbdce4f6a91d09f30f033e0a.tar.gz llvm-89fa06ba0f54cc90dbdce4f6a91d09f30f033e0a.tar.bz2 llvm-89fa06ba0f54cc90dbdce4f6a91d09f30f033e0a.tar.xz |
Switch-to-lookup tables: set threshold to 3 cases
There has been an old FIXME to find the right cut-off for when it's worth
analyzing and potentially transforming a switch to a lookup table.
The switches always have two or more cases. I could not measure any speed-up
by transforming a switch with two cases. A switch with three cases gets a nice
speed-up, and I couldn't measure any compile-time regression, so I think this
is the right threshold.
In a Clang self-host, this causes 480 new switches to be transformed,
and reduces the final binary size with 8 KB.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199294 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'lib/Transforms/Utils/SimplifyCFG.cpp')
-rw-r--r-- | lib/Transforms/Utils/SimplifyCFG.cpp | 8 |
1 files changed, 3 insertions, 5 deletions
diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp index 90f8847bc1..a30dcf2fe0 100644 --- a/lib/Transforms/Utils/SimplifyCFG.cpp +++ b/lib/Transforms/Utils/SimplifyCFG.cpp @@ -3684,11 +3684,9 @@ static bool SwitchToLookupTable(SwitchInst *SI, // GEP needs a runtime relocation in PIC code. We should just build one big // string and lookup indices into that. - // Ignore the switch if the number of cases is too small. - // This is similar to the check when building jump tables in - // SelectionDAGBuilder::handleJTSwitchCase. - // FIXME: Determine the best cut-off. - if (SI->getNumCases() < 4) + // Ignore switches with less than three cases. Lookup tables will not make them + // faster, so we don't analyze them. + if (SI->getNumCases() < 3) return false; // Figure out the corresponding result for each case value and phi node in the |