[LLVMdev] Query on llvm-gcc v/s llc/as

[Samuel Crow]

Hi Raghu,

I think the difference lies in the fact that llc has 4 as its maximum 
optimization level to allow link-time optimization.  If this is what you're 
noticing, then the second should be modified to "llc -O4 test.bc -o test.s".  I 
could be wrong though.

--Sam


----- Original Message ----
> From: Raghu Prabhakar <raghu at cs.ucla.edu>
> To: llvmdev at cs.uiuc.edu
> Sent: Fri, September 17, 2010 1:10:54 PM
> Subject: [LLVMdev] Query on llvm-gcc v/s llc/as
> 
> Hi all,
> 
> I have started using LLVM recently, and I have a basic question.  There are two 
>different ways in which an executable can be generated from a .c  file - by just 
>using llvm-gcc, and using a combination of llvm-gcc (to emit  bitcode .bc), llc 
>(to emit assembly code), as (to assemble the code) and  collect2 (linker). For 
>example, consider compiling test.c :
> 
> 1) llvm-gcc  -O3 test.c -o testExecutable
> 
> 2) llvm-gcc -O3 -emit-llvm -c test.c -o  test.bc
>    llc -O3 test.bc -o test.s
>    as test.s -o  test.o
>    gcc test.o -o testExecutable2
> 
> My question is, are the  two executables (testExecutable and testExecutable2) 
>basically the same ? Is  there any huge difference between the two processes 
>which will impact the  executable's running time ? I am asking this because I 
>compared the executables  by their wordcounts (wc -c), and there was a slight 
>difference. However, when I  timed the test runs, both performances were 
>identical.
> 
> Kindly help me out  on  this.
> 
> Thanks.
> Raghu.
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