[PATCH] D21291: [SimplifyCFG] Range reduce switches

James Molloy via llvm-commits llvm-commits at lists.llvm.org
Mon Jun 13 07:53:53 PDT 2016

jmolloy created this revision.
jmolloy added reviewers: mcrosier, sbaranga, sanjoy.
jmolloy added a subscriber: llvm-commits.
jmolloy set the repository for this revision to rL LLVM.

If a switch is sparse and all the cases (once sorted) are in arithmetic progression, we can extract the common factor out of the switch and create a dense switch. For example:

    switch (i) {
    case 5: ...
    case 9: ...
    case 13: ...
    case 17: ...

can become:

    if ( (i - 5) % 4 ) goto default;
    switch ((i - 5) / 4) {
    case 0: ...
    case 1: ...
    case 2: ...
    case 3: ...

The division and remainder operations could be costly so we only do this if the factor is a power of two. Dense switches can be lowered significantly better than sparse switches and can even be transformed into lookup tables.




-------------- next part --------------
A non-text attachment was scrubbed...
Name: D21291.60524.patch
Type: text/x-patch
Size: 5911 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20160613/fb2eb582/attachment.bin>

More information about the llvm-commits mailing list