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digitalmars.D.learn - -fPIC and 32-bit dmd.conf settings

reply Joseph Rushton Wakeling <joseph.wakeling webdrake.net> writes:
Hello folks,

The default dmd.conf settings for 64-bit environments include the 
-fPIC flag (for good reason), but the settings for 32-bit 
environments do not.  Any particular reason for this?

Thanks & best wishes,

     -- Joe
Apr 08 2017
next sibling parent reply Patrick Schluter <Patrick.Schluter bbox.fr> writes:
On Saturday, 8 April 2017 at 21:31:31 UTC, Joseph Rushton 
Wakeling wrote:

 Hello folks,

 The default dmd.conf settings for 64-bit environments include 
 the -fPIC flag (for good reason), but the settings for 32-bit 
 environments do not.  Any particular reason for this?

 Thanks & best wishes,

     -- Joe


32 bit modes support for PIC is painful because the RIP relative 
addressing mode does not exist. AMD introduced it when they added 
64 bit support in the Opteron processors. In 32 bit mode it was 
possible to generate PIC code but it required some ugly hacks 
(CALL 0) which broke some hardware optimization (the return 
address target cache) and adds a layer of indirection for shared 
libraries (GOT), which had the consequence of breaking the 
sharing, i.e. .so and .dll weren't really shared in 32 bit mode.
Here an in depth article that I haven't yet read but seem to be 
interesting on the subject 
https://www.technovelty.org/c/position-independent-code-and-x86-64-libraries.html
Of course, this has nothing to do with D per se.
Apr 09 2017
parent reply Joseph Rushton Wakeling <joseph.wakeling webdrake.net> writes:
On Sunday, 9 April 2017 at 07:12:51 UTC, Patrick Schluter wrote:

 32 bit modes support for PIC is painful because the RIP 
 relative addressing mode does not exist. AMD introduced it when 
 they added 64 bit support in the Opteron processors. In 32 bit 
 mode it was possible to generate PIC code but it required some 
 ugly hacks (CALL 0) which broke some hardware optimization (the 
 return address target cache) and adds a layer of indirection 
 for shared libraries (GOT), which had the consequence of 
 breaking the sharing, i.e. .so and .dll weren't really shared 
 in 32 bit mode.
 Here an in depth article that I haven't yet read but seem to be 
 interesting on the subject 
 https://www.technovelty.org/c/position-independent-code-and-x86-64-libraries.html
 Of course, this has nothing to do with D per se.


Thanks for the explanation.  TBH I find myself wondering whether 
`-fPIC` should be in the flags defined in dmd.conf _at all_ (even 
for 64-bit environments); surely that should be on request of 
individual project builds ... ?
Apr 09 2017
parent Stefan Koch <uplink.coder googlemail.com> writes:
On Sunday, 9 April 2017 at 09:23:07 UTC, Joseph Rushton Wakeling 
wrote:

 Thanks for the explanation.  TBH I find myself wondering 
 whether `-fPIC` should be in the flags defined in dmd.conf _at 
 all_ (even for 64-bit environments); surely that should be on 
 request of individual project builds ... ?


It's needed in order to compile working executables on debain & 
ubuntu.

b/c of PIE.
Apr 09 2017
prev sibling parent Patrick Schluter <Patrick.Schluter bbox.fr> writes:
On Saturday, 8 April 2017 at 21:31:31 UTC, Joseph Rushton 
Wakeling wrote:

 Hello folks,

 The default dmd.conf settings for 64-bit environments include 
 the -fPIC flag (for good reason), but the settings for 32-bit 
 environments do not.  Any particular reason for this?

 Thanks & best wishes,

     -- Joe


And here an article explaining the difference between PIC on 
32bit and 64 bit code

http://www.bailopan.net/blog/?p=50
Apr 09 2017