pprC: declare extern cmm primitives as functions, not data
Summary:
The commit fixes incorrect code generation of
integer-gmp package on ia64 due to C prototypes mismatch.
Before the patch prototypes for "foreign import prim" were:
StgWord poizh[];
After the patch they became:
StgFunPtr poizh();
Long story:
Consider the following simple example:
{-# LANGUAGE MagicHash, GHCForeignImportPrim, UnliftedFFITypes #-}
module M where
import GHC.Prim -- Int#
foreign import prim "poizh" poi# :: Int# -> Int#
Before the patch unregisterised build generated the
following 'poizh' reference:
EI_(poizh); /* StgWord poizh[]; */
FN_(M_poizh_entry) {
// ...
JMP_((W_)&poizh);
}
After the patch it looks this way:
EF_(poizh); /* StgFunPtr poizh(); */
FN_(M_poizh_entry) {
// ...
JMP_((W_)&poizh);
}
On ia64 it leads to different relocation types being generated:
incorrect one:
addl r14 = @ltoffx(poizh#)
ld8.mov r14 = [r14], poizh# ; r14 = address-of 'poizh#'
correct one:
addl r14 = @ltoff(@fptr(poizh#)), gp ; r14 = address-of-thunk 'poizh#'
ld8 r14 = [r14]
'@fptr(poizh#)' basically instructs assembler to creates
another obect consisting of real address to 'poizh' instructions
and module address. That '@fptr' object is used as a function "address"
This object is different for every module referencing 'poizh' symbol.
All indirect function calls expect '@fptr' object. That way
call site reads real destination address and set destination
module address in 'gp' register from '@fptr'.
Signed-off-by:
Sergei Trofimovich <slyfox@gentoo.org>
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mentioned in issue #8819 (closed)
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mentioned in commit d82f5925
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mentioned in commit ab76b099
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