[GHC] #14452: `-optc-O3` getting shadowed by automatically injected -O flags

12 Nov 2017

      #14452: `-optc-O3` getting shadowed by automatically injected -O flags
-------------------------------------+-------------------------------------
           Reporter:  hvr            |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Consider the following example:

 {{{#!hs
 {-# LANGUAGE CApiFFI #-}

 {-# OPTIONS_GHC -optc-O3 #-}

 module M where

 foreign import capi unsafe "stdlib.h exit" c_exit :: Int -> IO ()
 }}}

 However, the `-optc-O3` flag has no effect, unless `ghc` is called without
 any `-O` flags, or with `-O0`.

 Here's the resulting C compiler invocations for different `-O` flags:

 {{{
 $ ghc -fforce-recomp -v -c  m.hs |& grep O3
 gcc -fno-stack-protector -DTABLES_NEXT_TO_CODE -O3 -x c
 /tmp/ghc14023_0/ghc_2.c -o /tmp/ghc14023_0/ghc_3.s -Wimplicit -S -include
 /opt/ghc/8.2.1/lib/ghc-8.2.1/include/ghcversion.h -I.
 -I/opt/ghc/8.2.1/lib/ghc-8.2.1/base-4.10.0.0/include
 -I/opt/ghc/8.2.1/lib/ghc-8.2.1/integer-gmp-1.0.1.0/include
 -I/opt/ghc/8.2.1/lib/ghc-8.2.1/include
 }}}

 {{{
 $ ghc -fforce-recomp -O0 -v -c  m.hs |& grep O3
 gcc -fno-stack-protector -DTABLES_NEXT_TO_CODE -O3 -x c
 /tmp/ghc14045_0/ghc_2.c -o /tmp/ghc14045_0/ghc_3.s -Wimplicit -S -include
 /opt/ghc/8.2.1/lib/ghc-8.2.1/include/ghcversion.h -I.
 -I/opt/ghc/8.2.1/lib/ghc-8.2.1/base-4.10.0.0/include
 -I/opt/ghc/8.2.1/lib/ghc-8.2.1/integer-gmp-1.0.1.0/include
 -I/opt/ghc/8.2.1/lib/ghc-8.2.1/include
 }}}

 {{{
 $ ghc -fforce-recomp -O1 -v -c  m.hs |& grep O3
 gcc -fno-stack-protector -DTABLES_NEXT_TO_CODE -O3 -x c
 /tmp/ghc14073_0/ghc_2.c -o /tmp/ghc14073_0/ghc_3.s -Wimplicit -S -O
 -include /opt/ghc/8.2.1/lib/ghc-8.2.1/include/ghcversion.h -I.
 -I/opt/ghc/8.2.1/lib/ghc-8.2.1/base-4.10.0.0/include
 -I/opt/ghc/8.2.1/lib/ghc-8.2.1/integer-gmp-1.0.1.0/include
 -I/opt/ghc/8.2.1/lib/ghc-8.2.1/include
 }}}

 {{{
 $ ghc -fforce-recomp -O2 -v -c  m.hs |& grep O3
 gcc -fno-stack-protector -DTABLES_NEXT_TO_CODE -O3 -x c
 /tmp/ghc14093_0/ghc_2.c -o /tmp/ghc14093_0/ghc_3.s -Wimplicit -S -O2
 -include /opt/ghc/8.2.1/lib/ghc-8.2.1/include/ghcversion.h -I.
 -I/opt/ghc/8.2.1/lib/ghc-8.2.1/base-4.10.0.0/include
 -I/opt/ghc/8.2.1/lib/ghc-8.2.1/integer-gmp-1.0.1.0/include
 -I/opt/ghc/8.2.1/lib/ghc-8.2.1/include
 }}}

 {{{
 $ ghc -fforce-recomp -O3 -v -c  m.hs |& grep O3
 gcc -fno-stack-protector -DTABLES_NEXT_TO_CODE -O3 -x c
 /tmp/ghc14119_0/ghc_2.c -o /tmp/ghc14119_0/ghc_3.s -Wimplicit -S -O2
 -include /opt/ghc/8.2.1/lib/ghc-8.2.1/include/ghcversion.h -I.
 -I/opt/ghc/8.2.1/lib/ghc-8.2.1/base-4.10.0.0/include
 -I/opt/ghc/8.2.1/lib/ghc-8.2.1/integer-gmp-1.0.1.0/include
 -I/opt/ghc/8.2.1/lib/ghc-8.2.1/include
 }}}

 To summarise, here's the resulting effective `-O`-level passed to `gcc`:

 ||= GHC invocation =||= GCC invocation =||= Effective C `-O`-level =||
 || `ghc`  || `gcc -O3` || `-O3` ||
 || `ghc -O0`  || `gcc -O3` || `-O3` ||
 || `ghc -O1`  || `gcc -O3 -O1` || `-O1` ||
 || `ghc -O2`  || `gcc -O3 -O2` || `-O2` ||
 || `ghc -O3`  || `gcc -O3 -O2` || `-O2` ||

 Consequently, the only way to have C code compiled with `-O3` is to force
 GHC into `-O0` mode; this is obviously not ideal, as it easily kills any
 benefit you'd gain from passing `-O3` to the C compiler... ;-)

 I see a few alternatives on how to resolve this:

  1. Have GHC recognise `-optc-O[0-9]` and suppress automatically injecting
 any `-O<n>` of its own
  2. Have GHC inject its automatic `-O`-flags before user-provided `-optc`
 flags
  3. Have GHC inject `-optc` flags as late as possible on the C compiler
 command-line
  4. Implement a new variant of the `-optc` flag which injects its flags as
 late as possible on the C compiler command-line

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14452>
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