#15929: Explore whether adding XRay attributes to LLVM IR is worthwhile -------------------------------------+------------------------------------- Reporter: mpickering | Owner: (none) Type: task | Status: new Priority: normal | Milestone: 8.6.3 Component: Compiler | Version: 8.6.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by mpickering): I tried this out. The diff to add support was very simple. https://github.com/mpickering/ghc/commit/42c3df674e684fc33bcd472db8b55ace3d4... I then tried to compile the following program and to enable the XRay trace. {{{ module Main where main :: IO () main = print "abc" >> print (f 1000) >> print (g 1000) >> print (qux B) data T = A | B deriving Show f 0 = 1 f x = f (x -1) + 1 g 1 = 0 g x = g (x - 1) + f (x - 1) {-# NOINLINE qux #-} qux A = A qux B = qux A }}} The first thing to note was that it was necessary to use `clang-7.0.0` which I achieved by using this diff to `ghc.nix`. https://gist.github.com/mpickering/f1317ea6aac955eb9ecb1e2e1ef0fba7 Then I compiled with the following sequence of commands: {{{ inplace/bin/ghc-stage2 -fllvm -ddump-llvm llvm.hs -opta="-v" -pgma=clang -pgmc=clang -pgml=clang -optl="-fxray-instrument" -optl="-fxray- instruction-threshold=1" -O2 -fforce-recomp XRAY_OPTIONS="patch_premain=true xray_mode=xray-basic verbosity=1" ./llvm }}} Then the log file was analysed using the following command {{{ llvm-xray account -sort=sum -sortorder=dsc -instr_map ./llvm xray- log.llvm.XEyh0y }}} Which produced the very unhelpful profiling output. {{{ Functions with latencies: 7 funcid count [ min, med, 90p, 99p, max] sum function 7 18 [ 0.000006, 0.000006, 0.000021, 0.000040, 0.000040] 0.000175 <invalid>:0:0: @(422fd0) 5 14 [ 0.000006, 0.000006, 0.000014, 0.000021, 0.000021] 0.000110 <invalid>:0:0: @(422e90) 10 8 [ 0.000006, 0.000006, 0.000042, 0.000042, 0.000042] 0.000104 <invalid>:0:0: @(4231a8) 8 10 [ 0.000006, 0.000006, 0.000040, 0.000040, 0.000040] 0.000096 <invalid>:0:0: @(423070) 6 14 [ 0.000006, 0.000006, 0.000006, 0.000006, 0.000006] 0.000086 <invalid>:0:0: @(422f30) 11 13 [ 0.000006, 0.000006, 0.000006, 0.000008, 0.000008] 0.000082 <invalid>:0:0: @(423260) 9 7 [ 0.000006, 0.000006, 0.000014, 0.000014, 0.000014] 0.000055 <invalid>:0:0: @(423110) }}} Should be easy now for someone else more familiar with LLVM to investigate. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15929#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#15929: Explore whether adding XRay attributes to LLVM IR is worthwhile -------------------------------------+------------------------------------- Reporter: mpickering | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.2 Resolution: | Keywords: newcomer Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by mpickering): I investigated some more and get the basic support working. None of the call stack stuff is useful for GHC as every call is a tail call so the call hierarchy is completely flat. Here is the branch: https://github.com/mpickering/ghc/tree/llvm-xray See this thread on llvm-devs for some of the problems and suggestions of the LLVM devs: http://lists.llvm.org/pipermail/llvm- dev/2018-December/128184.html Here's an example comment I was using to compile which correctly instrumented a binary. {{{ _build/stage1/bin/ghc -fllvm -ddump-llvm llvm.hs -opta="-v" -pgma=clang -pgmc=clang -pgml=clang -optl="-fxray-instrument" -optl="-fxray- instruction-threshold=1" -optlo="-O0" -O2 -g3 -fforce-recomp nofib/real/fulsom/Main.hs -inofib/real/fulsom/ }}} Problems 1. You have to pass `-O0` to `opt` as otherwise the LLVM verifier complains about our debugging information. See the email list for more information about this. 2. It didn't seem like every call was actually counted in some simple tests but I didn't look too closely. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15929#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler