`-optc-O3` getting shadowed by automatically injected -O flags

Consider the following example:

{-# 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

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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