Avoid unnecessary blocks and branches #7941
Closed
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Currently, all closures have an llenv block to load values from the captured environment, but for closure that don't actually capture anything, that block is useless and can be skipped.
The nested with_cond calls each introduce a "next" block, with the inner one just jumping to the outer one.
Currently, the helper functions in the "build" module can only append at the end of a block. For certain things we'll want to be able to insert code at arbitrary locations inside a block though. Although can we do that by directly calling the LLVM functions, that is rather ugly and means that somethings need to be implemented twice. Once in terms of the helper functions and once in terms of low level LLVM functions. Instead of doing that, we should provide a Builder type that provides low level access to the builder, and which can be used by both, the helper functions in the "build" module, as well larger units of abstractions that combine several LLVM instructions.
These blocks were required because previously we could only insert instructions at the end of blocks, but we wanted to have all allocas in one place, so they can be collapse. But now we have "direct" access the the LLVM IR builder and can position it freely. This allows us to use the same trick that clang uses, which means that we insert a dummy "marker" instruction to identify the spot at which we want to insert allocas. We can then later position the IR builder at that spot and insert the alloca instruction, without any dedicated block. The block for loading the closure environment can now also go away, because the function context now provides the toplevel block, and the translation of the loading happens first, so that's good enough. Makes the LLVM IR a bit more readable, saving a bunch of branches in the unoptimized code, which benefits unoptimized builds.
bors
added a commit
that referenced
this pull request
Jul 22, 2013
These changes remove unnecessary basic blocks and the associated branches from the LLVM IR that we emit. Together, they reduce the time for unoptimized builds in stage2 by about 10% on my box.
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
These changes remove unnecessary basic blocks and the associated branches from
the LLVM IR that we emit. Together, they reduce the time for unoptimized builds
in stage2 by about 10% on my box.