As I have explained in another thread, I am in the process of porting portions of newlib to LLVM. The target system has no operating system and a custom processor which may be changed from compilation to compilation. We intend to use LLVM as the front end and generate target specific code from LLVM byte code. For various reasons (whole program optimization being one of them), it would seem to make sense to do most of linking at the byte code level. I succesfully built a byte code version of newlib, so I tried to use it like this: $ llvm-link hello.bc $NEWLIB/libc.a -o linked.bc but apparently llvm-link only understands byte code files. Does it even make sense to try to use llvm-link as a ld replacement this way? Or am I missing something else that could be used? The other option is of course to link all of libc into one big byte code file, and use llvm-link like this: $ llvm-link hello.bc $NEWLIB/libc.bc -o linked.bc The only concern here is that this brings in all of libc, but I suppose it should be easy enough to run dead code elimination on linked.bc to shake out the unused bits? -- Pertti
To follow up on my own post:> The only concern here is that this brings in all of libc, but I > suppose it should be easy enough to run dead code elimination on > linked.bc to shake out the unused bits?Cursory browsing of LLVM optimization passes did not turn up anything directly applicable, at least "opt -adce" and "opt -globaldce" did not reduce the size of the byte code file much. What I am looking for is a transformation that would take a starting point in a byte code file (e.g. %main()), determine the transitive closure of the functions and globals that can be reached from there, and drop everything else. In the Lisp world it would be called a tree shaker, I don't know how you guys would call it. If such a pass does not exist, it would seem like a perfect candidate for playing around with pass writing, so I would be happy to implement it if need be. -- Pertti
On Wed, 15 Nov 2006, [ISO-8859-1] Pertti Kellom�ki wrote:> To follow up on my own post: >> The only concern here is that this brings in all of libc, but I >> suppose it should be easy enough to run dead code elimination on >> linked.bc to shake out the unused bits? > > Cursory browsing of LLVM optimization passes did not turn up > anything directly applicable, at least "opt -adce" and "opt -globaldce" > did not reduce the size of the byte code file much. > > What I am looking for is a transformation that would take a > starting point in a byte code file (e.g. %main()), determine > the transitive closure of the functions and globals that can be reached > from there, and drop everything else. In the Lisp world it would > be called a tree shaker, I don't know how you guys would call it. > > If such a pass does not exist, it would seem like a perfect candidate > for playing around with pass writing, so I would be happy to > implement it if need be.try running opt -internalize -globaldce. In particular, the internalize pass will tell LLVM that you have the "whole program" (i.e. it can mark all functions as static other than main). This allows the globaldce pass to be more aggressive. -Chris -- http://nondot.org/sabre/ http://llvm.org/
On Wed, 15 Nov 2006, [ISO-8859-1] Pertti Kellom�ki wrote:> I succesfully built a byte code version of newlib, so I tried to > use it like this: > > $ llvm-link hello.bc $NEWLIB/libc.a -o linked.bc > > but apparently llvm-link only understands byte code files.Right, llvm-link is the low level bytecode linking interface, not really useful for this sort of use.> Does it even make sense to try to use llvm-link as a ld replacement > this way? Or am I missing something else that could be used?The 'gccld' tool understands archives. Try: gccld hello.bc .../libc.a -o linked.bc -link-as-library "-link-as-library" causes it to emit a .bc file instead of trying to link an app. -Chris -- http://nondot.org/sabre/ http://llvm.org/
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