#8321: improve basic block layout on LLVM backend by predicting stack/heap checks -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: rwbarton Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (LLVM) | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): -------------------------------------+------------------------------------- Comment (by Ben Gamari <ben@…>): In [changeset:"20fb781ed1825578c5428ff4ae408be034c6a1d8/ghc" 20fb781e/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="20fb781ed1825578c5428ff4ae408be034c6a1d8" LLVM generate llvm.expect for conditional branches This patch adds likeliness annotations to heap and and stack checks and modifies the llvm codegen to recognize those to help it generate better code. So with this patch ``` ... if ((Sp + 8) - 24 < SpLim) (likely: False) goto c23c; else goto c23d; ... ``` roughly generates: ``` %ln23k = icmp ult i64 %ln23j, %SpLim_Arg %ln23m = call ccc i1 (i1, i1) @llvm.expect.i1( i1 %ln23k, i1 0 ) br i1 %ln23m, label %c23c, label %c23d ``` Note the call to `llvm.expect` which denotes the expected result for the comparison. Test Plan: Look at assembler code with and without this patch. If the heap-checks moved out of the way we are happy. Reviewers: austin, simonmar, bgamari Reviewed By: bgamari Subscribers: michalt, thomie Differential Revision: https://phabricator.haskell.org/D2688 GHC Trac Issues: #8321 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8321#comment:7> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler