[GHC] #15929: Explore whether adding XRay attributes to LLVM IR is worthwhile

[GHC]

#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:                    |
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Comment (by mpickering):

 I tried this out. The diff to add support was very simple.

 https://github.com/mpickering/ghc/commit/42c3df674e684fc33bcd472db8b55ace3d4385ca

 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>
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