diff options
| -rw-r--r-- | compiler/cmm/CmmContFlowOpt.hs | 17 |
1 files changed, 9 insertions, 8 deletions
diff --git a/compiler/cmm/CmmContFlowOpt.hs b/compiler/cmm/CmmContFlowOpt.hs index b816a0c674..f39ab3c62b 100644 --- a/compiler/cmm/CmmContFlowOpt.hs +++ b/compiler/cmm/CmmContFlowOpt.hs @@ -56,25 +56,26 @@ cmmCfgOptsProc _ top = top -- This optimisation does three things: -- -- - If a block finishes with an unconditional branch, then we may --- be able to concatenate the block it points to and remove the --- branch. We do this either if the destination block is small --- (e.g. just another branch), or if this is the only jump to --- this particular destination block. +-- be able to duplicate the block it points to and remove the +-- branch. We do this if either +-- a) the destination block is small (e.g. just another branch), or +-- b) this is the only jump to this particular destination block. -- -- - If a block finishes in a call whose continuation block is a -- goto, then we can shortcut the destination, making the --- continuation block the destination of the goto. +-- destination of the goto into the continuation. E.g. +-- call g returns to L ==> call g returns to M +-- L: goto M M: ...blah... +-- M: ...blah... -- (but see Note [shortcut call returns]) -- --- - removes any unreachable blocks from the graph. This is a side +-- - Remove any unreachable blocks from the graph. This is a side -- effect of starting with a postorder DFS traversal of the graph --- -- Both transformations are improved by working from the end of the -- graph towards the beginning, because we may be able to perform many -- shortcuts in one go. - -- We need to walk over the blocks from the end back to the -- beginning. We are going to maintain the "current" graph -- (BlockEnv CmmBlock) as we go, and also a mapping from BlockId |