llvm dev - May 2017 - Printing MachineBasicBlock in an order that follows the CFG of the MachineFunction

Hello.
     Is there a possibility to generate the ASM program with MachineBasicBlocks
in an
order that strictly respects the predecessors of each BasicBlock (BB)?
     I ask because currently there are some cases where actually the BBs are
printed in
the .s file in a somewhat wrong order - for example, 1 BB does not follow
immediately
after its first predecessor although it should.
     I would like to mention I am using LLVM checked out from SVN in Jul 2016,
but I guess
this problem persists in the latest revision.

     I guess I can modify the current implementation to change the order of the
BBs as
they are created in the MachineFunction class, s.t. when we later iterate over
the BBs to
get the right order.
     Would this be a safe change? If not I can generate the right order when
LLVM prints
the ASM file, for example in method EmitFunctionBody().

   Thank you,
     Alex

> On May 17, 2017, at 5:11 PM, Alex Susu via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
>  Hello.
>    Is there a possibility to generate the ASM program with
MachineBasicBlocks in an order that strictly respects the predecessors of each
BasicBlock (BB)?
>    I ask because currently there are some cases where actually the BBs are
printed in the .s file in a somewhat wrong order - for example, 1 BB does not
follow immediately after its first predecessor although it should.
Not sure I fully understand you there, but with loops involved you simply cannot
always print all predecessors before the block itself.
>    I would like to mention I am using LLVM checked out from SVN in Jul
2016, but I guess this problem persists in the latest revision.
> 
>    I guess I can modify the current implementation to change the order of
the BBs as they are created in the MachineFunction class, s.t. when we later
iterate over the BBs to get the right order.
>    Would this be a safe change? If not I can generate the right order when
LLVM prints the ASM file, for example in method EmitFunctionBody().
As long as you are in IR you could modify the blocks to be in an arbitrary order
(RPO probably gets you closest to what you want).

CodeGen however reorders block for performance anyway (see BranchFolding). This
however only works if your target implements TargetInstrInfo::analyzeBranch() 
(and whatever else BranchFolding uses).

- Matthias