[llvm-dev] Need help with code generation

Sat Mar 19 15:15:13 PDT 2016

@james
Yeah for code generation I figured out that clang doesn't actually use llc, and I already started reading its code to see how it works.
For the ld, there's not an "helper" in the llvm library that calls it, is there?
By the way, I thought about calling ld with things like execl() or std::system(), I don't know if it's a good idea, I'm always afraid there are better ways than mine!

@mats
Yea, I haven't used C's system calls in my own code yet but if I just have to declare the function puts in the IR modules (e.g.: putc) and then link against libc, am I right?
Should I use basic C functions such as putc() and getc(), in my runtime library or is there a more efficient way to set up my runtime library?

> On Mar 19, 2016, at 9:58 PM, mats petersson <mats at planetcatfish.com> wrote:
> 
> If you plan on calling C runtime library functions, you probably want to do what I did:
> Cheat, and make a libruntime.a (with C functions to do stuff your compiler can't do natively) and then link that using clang or gcc. 
> 
> https://github.com/Leporacanthicus/lacsap/blob/master/binary.cpp#L124
> 
> At some point, I plan to replace my runtime library with native Pascal code, at which point I will be able to generate the ELF binary straight from my compiler without the runtime library linking in the C runtime library, but that's not happening anytime real soon. Getting the compiler to compile v5 of Wirth's original Pascal compiler is higher on the list... :)
> 
> --
> Mats
> 
>> On 19 March 2016 at 20:51, James Molloy via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> Hi Lorenzo,
>> 
>> Clang doesn't call llc; LLVM is compiled into Clang. Clang does call the system linker though.
>> 
>> Making your compiler generate *object* code is very simple. Making it fixup that object code and execute it in memory (JIT style) is also simple. Linking it properly and creating a fixed up ELF file is less simple. For that, you need to compile to object (using addPassesToEmitFile() - see llc.cpp) then invoke a linker. Getting that command line right can be quite difficult.
>> 
>> Rafael, This would be a good usecase for LLD as a library. I heard that this is is an explicit non-goal, which really surprised me. Is that indeed the case?
>> 
>> Cheers,
>> 
>> James
>> 
>>> On Sat, 19 Mar 2016 at 13:32 Lorenzo Laneve via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>> I'd like to make my compiler independent, just like Clang. Doesn't Clang call llc and then system's ld by itself? I don't want my compiler to depend by any other program.
>>> I guess there will be a class in the llvm library that generates the object files based on the system's triple and data layout, and then call the system's ld?
>>> 
>>>> On Mar 19, 2016, at 11:48 AM, Bruce Hoult <bruce at hoult.org> wrote:
>>>> 
>>>> If you've created a .bc or a .ll file then the simplest thing is to just give it to clang exactly the same as you would for a .c file. Clang will just Do The Right Thing with it.
>>>> 
>>>> If you don't want to link, then pass flags such as -c to clang as usual.
>>>> 
>>>> e.g.
>>>> 
>>>> ---- hello.ll ----
>>>> declare i32 @puts(i8*)
>>>> @str = constant [12 x i8] c"Hello World\00"
>>>> 
>>>> define i32 @main() {
>>>>   %1 = call i32 @puts(i8* getelementptr inbounds ([12 x i8]* @str, i64 0, i64 0))
>>>>   ret i32 0
>>>> }
>>>> ----------------
>>>> 
>>>> $ clang hello.ll -o hello && ./hello
>>>> warning: overriding the module target triple with x86_64-apple-macosx10.10.0
>>>> 1 warning generated.
>>>> Hello World
>>>> 
>>>> 
>>>>> On Sat, Mar 19, 2016 at 3:03 AM, Lorenzo Laneve via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>>>> I wrote my compiler and now it generates LLVM IR modules. Now i’d like to go ahead and make object file and then executable, just like clang does.
>>>>> 
>>>>> What should I have to use to create the object files? and then how do I call the ld? (not llvm-ld, I want my compiler to work like Clang and I read that Clang doesn’t use llvm-ld).
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