ld with gold-plugin can do this?

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1

Hi,all

I have installed the gold-plugin and it can run correctly,but I still
have another two questions:

1.Can gold-plugin generate a bitcode file:

From the document and also my experience on using it,"ld -plugin...."

will only generate an executable by link several LLVM bitcode file
together,but how to make it generate a whole-program bitcode file?(even
archieve it manually is OK)

2.llvm-gcc -c will stop the linker to run:

I have been trying to generate a whole-program bitcode file,but when I
use CFLAGS="-emit-llvm -c" to compile source files into LLVM bitcode
file,llvm-gcc will not call "ld" to link modules together,so that I
cannot link these modules together automatically.

How to overcome this trouble?

Thank you !Best regards!

Nan

2

zhunan wrote:

Hi,all

I have installed the gold-plugin and it can run correctly,but I still
have another two questions:

1.Can gold-plugin generate a bitcode file:

No. The task of linking many .bc files into a single .bc file falls on llvm-ld and llvm-link.

A common followup question is "but how do I link native libraries into my .bc file". You don't. A .bc file is llvm ir, you can't put a native binary library into a .bc (barring sticking it in as a string, etc).

The build then looks like:

a) 'llvm-gcc -c -flo -O2' to generate the .bc files.
b) 'llvm-ld' to combine them into a single .bc. No, not a .so nor a .a.
c) 'llc' to turn your combined .bc into a .s
d) 'as' to turn your .s into a .o
e) 'ld' to turn your .o into a .so or final executable. This is the step where you get to specify all the native libraries to link with.

You can use 'gcc' to merge steps d and e (it just runs as and ld for you). Or you can use the gold plugin to merge steps b through e, but with the added benefit that it will optimize slightly better. See the llvm LTO documentation on why.

From the document and also my experience on using it,"ld -plugin...."

will only generate an executable by link several LLVM bitcode file
together,but how to make it generate a whole-program bitcode file?(even
archieve it manually is OK)

2.llvm-gcc -c will stop the linker to run:

I have been trying to generate a whole-program bitcode file,but when I
use CFLAGS="-emit-llvm -c" to compile source files into LLVM bitcode
file,llvm-gcc will not call "ld" to link modules together,so that I
cannot link these modules together automatically.

How to overcome this trouble?

You get to modify your program's build system. Things like 'configure' work by trying to compile and run small programs. You can't run a .bc file.

Nick

3

A common followup question is “but how do I link native libraries into
my .bc file”. You don’t. A .bc file is llvm ir, you can’t put a native
binary library into a .bc (barring sticking it in as a string, etc).

The build then looks like:

a) ‘llvm-gcc -c -flo -O2’ to generate the .bc files.
b) ‘llvm-ld’ to combine them into a single .bc. No, not a .so nor a .a.
c) ‘llc’ to turn your combined .bc into a .s
d) ‘as’ to turn your .s into a .o
e) ‘ld’ to turn your .o into a .so or final executable. This is the step
where you get to specify all the native libraries to link with.

You can use ‘gcc’ to merge steps d and e (it just runs as and ld for
you). Or you can use the gold plugin to merge steps b through e, but
with the added benefit that it will optimize slightly better. See the
llvm LTO documentation on why.

[Sanjiv] - This is what PIC16 does. llvm-ld has an option called ‘-b’ that you can
use to specify the output bitcode file name. Unfortunately, llvm-ld always tries to
generate a native executable (or a shell script) as well and currently there is no way to disable that.
Maybe an option like ‘-no-native’ will be come handy.

  • Sanjiv
4

Yes there is, it's -link-as-library.

Nick

5

Nick Lewycky wrote:

6

Sanjiv Gupta wrote:

Nick Lewycky wrote:

A common followup question is "but how do I link native libraries into
my .bc file". You don't. A .bc file is llvm ir, you can't put a native
binary library into a .bc (barring sticking it in as a string, etc).

The build then looks like:

a) 'llvm-gcc -c -flo -O2' to generate the .bc files.
b) 'llvm-ld' to combine them into a single .bc. No, not a .so nor a .a.
c) 'llc' to turn your combined .bc into a .s
d) 'as' to turn your .s into a .o
e) 'ld' to turn your .o into a .so or final executable. This is the step
where you get to specify all the native libraries to link with.

You can use 'gcc' to merge steps d and e (it just runs as and ld for
you). Or you can use the gold plugin to merge steps b through e, but
with the added benefit that it will optimize slightly better. See the
llvm LTO documentation on why.

[Sanjiv] - This is what PIC16 does. llvm-ld has an option called '-b' that you can
use to specify the output bitcode file name. Unfortunately, llvm-ld always tries to
generate a native executable (or a shell script) as well and currently there is no way to disable that.
Maybe an option like '-no-native' will be come handy.

Yes there is, it's -link-as-library.

Nick

Well, -link-as-library is not quite same as a building a final executable .bc.

.bc files aren't executable so I'm not entirely sure what you mean. 'llvm-ld -link-as-library' takes a group of .bc files and merges them before running the link-time optimizations over them, then emits the result as a .bc file.

The set of optimizations run is also different.

How so?

Checking llvm-ld's source, the only possible difference I can see is that passing -link-as-library will cause llvm-ld to ignore the -post-link-opts flag which almost certainly doesn't matter because you were almost certainly not using it.

Nick

7

Nick Lewycky wrote:

Sanjiv Gupta wrote:

Nick Lewycky wrote:

A common followup question is "but how do I link native libraries into
my .bc file". You don't. A .bc file is llvm ir, you can't put a native
binary library into a .bc (barring sticking it in as a string, etc).

The build then looks like:

a) 'llvm-gcc -c -flo -O2' to generate the .bc files.
b) 'llvm-ld' to combine them into a single .bc. No, not a .so nor a .a.
c) 'llc' to turn your combined .bc into a .s
d) 'as' to turn your .s into a .o
e) 'ld' to turn your .o into a .so or final executable. This is the step
where you get to specify all the native libraries to link with.

You can use 'gcc' to merge steps d and e (it just runs as and ld for
you). Or you can use the gold plugin to merge steps b through e, but
with the added benefit that it will optimize slightly better. See the
llvm LTO documentation on why.

[Sanjiv] - This is what PIC16 does. llvm-ld has an option called '-b' that you can
use to specify the output bitcode file name. Unfortunately, llvm-ld always tries to
generate a native executable (or a shell script) as well and currently there is no way to disable that.
Maybe an option like '-no-native' will be come handy.

Yes there is, it's -link-as-library.

Nick

Well, -link-as-library is not quite same as a building a final executable .bc.

.bc files aren't executable so I'm not entirely sure what you mean. 'llvm-ld -link-as-library' takes a group of .bc files and merges them before running the link-time optimizations over them, then emits the result as a .bc file.

The set of optimizations run is also different.

How so?

Checking llvm-ld's source, the only possible difference I can see is that passing -link-as-library will cause llvm-ld to ignore the -post-link-opts flag which almost certainly doesn't matter because you were almost certainly not using it.

Nick

if (LinkAsLibrary) {
      ...
      if (TheLinker.LinkInFiles(Files))
      ...
else {
     .....
      // Link all the items together
      if (TheLinker.LinkInItems(Items, NativeLinkItems) )

I don't think that LinkInFiles () and LinkInItems () do the same thing.

- Sanijv