llvm dev - May 2005 - [LLVMdev] SSA in the Front End

Hi,

I have been looking into the code that generates the LLVM assembly in the LLVM
front end, but I am
not very sure if at the time that the llvm_c_expand_body_1 function is called,
the SSA form was
already constructed (each definition dominates all the uses). Can somebody
please tell me?

Thanks

Ricardo wrote:
> Hi,
> 
> I have been looking into the code that generates the LLVM assembly in the
LLVM front end, but I am
> not very sure if at the time that the llvm_c_expand_body_1 function is
called, the SSA form was
> already constructed (each definition dominates all the uses). Can somebody
please tell me?
The LLVM GCC frontend does not translate variables directly into LLVM 
virtual regsiters (which must be in SSA form).

Instead, the LLVM GCC frontend simply uses a memory location for each 
variable (e.g. a local variable is allocated via the LLVM alloca 
instruction).  If you use llvm-gcc -S -o file.ll file.c, you can see the 
code that llvm-gcc generates.

Next, gccas (after assembling the LLVM assembly file) runs a pass called 
mem2reg.  This pass finds memory locations that can be promoted to LLVM 
virtual registers and promotes them into registers.  The following 
commands will allow you to see the change:

llvm-as -o file.bc file.ll
opt -mem2reg -o newfile.bc file.bc
llvm-dis -o newfile.ll newfile.bc

If you compare newfile.ll to file.ll, you'll see what the mem2reg 
transformation does.

Using this technique, language frontends don't need to worry about 
maintaining SSA for variables that it generates.  It simply generates 
them as memory locations and the LLVM optimizer fixes it up into more 
efficient code.

-- John T.
> 
> Thanks
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev

-- 
John T. Criswell
Research Programmer
University of Illinois at Urbana-Champaign
"It's today!" said Piglet. "My favorite day," said Pooh.

Thanks for the explanation. It's more clear now

The only thing that seems strange is that in the function
llvm_expand_shortcircuit_truth_expr in
the front end, there is the creation of a PHI instruction. If there is no SSA
yet, why do you do
that?

Thanks in advance




--- John Criswell <criswell at cs.uiuc.edu> wrote:
> Ricardo wrote:
> > Hi,
> > 
> > I have been looking into the code that generates the LLVM assembly in
the LLVM front end, but
> I am
> > not very sure if at the time that the llvm_c_expand_body_1 function is
called, the SSA form
> was
> > already constructed (each definition dominates all the uses). Can
somebody please tell me?
> 
> The LLVM GCC frontend does not translate variables directly into LLVM 
> virtual regsiters (which must be in SSA form).
> 
> Instead, the LLVM GCC frontend simply uses a memory location for each 
> variable (e.g. a local variable is allocated via the LLVM alloca 
> instruction).  If you use llvm-gcc -S -o file.ll file.c, you can see the 
> code that llvm-gcc generates.
> 
> Next, gccas (after assembling the LLVM assembly file) runs a pass called 
> mem2reg.  This pass finds memory locations that can be promoted to LLVM 
> virtual registers and promotes them into registers.  The following 
> commands will allow you to see the change:
> 
> llvm-as -o file.bc file.ll
> opt -mem2reg -o newfile.bc file.bc
> llvm-dis -o newfile.ll newfile.bc
> 
> If you compare newfile.ll to file.ll, you'll see what the mem2reg 
> transformation does.
> 
> Using this technique, language frontends don't need to worry about 
> maintaining SSA for variables that it generates.  It simply generates 
> them as memory locations and the LLVM optimizer fixes it up into more 
> efficient code.
> 
> -- John T.
> 
> > 
> > Thanks
> > 
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev
> 
> 
> -- 
> John T. Criswell
> Research Programmer
> University of Illinois at Urbana-Champaign
> "It's today!" said Piglet. "My favorite day," said
Pooh.
> 
>