llvm dev - Feb 2020 - Custom BR instruction emitted in wrong order

Hello,

To play around with, I duplicated the branch instruction implementation in LLVM
and Clang under the opcode ‘fbr’ as opposed to regular ‘br’. I accept LLVM to
accept the new opcode and have Clang produce ‘fbr’ branches instead of the
normal branches. But the place of ‘if.then' and ‘if.else' in the
instruction is switch. Instead of if.then occurring first, if.else is occurring
first. For example:

  The normal conditional branch instructions would be: "br i1 %cmp, label
%if.then, label %if.else”
  But if I force Clang to produce ’fbr’ instruction, it becomes: “fbr i1 %cmp,
label %if.else, label %if.then” (the condition doesn’t inverse either)

This is causing the output to the wrong since the right and wrong block are
switched. I traced it down to the generation of the custom branch instruction to
Instructions.cpp (it’s the same as BranchstInst class but just the opcode
changed) and dumping all the information once it’s being created. Everything is
in order until it returns back from FBranchInst::Create (in IRBuilder.h), which
is where the if.then and if.else are being switch. I have no idea what’s causing
it to flip, and what makes it weird is that if I pass the .ll file to opt (or
back it back into Clang with -emit-llvm option), the new output file has the
block order flipped back (so if.then is first and following that is if.else).
They flip to the wrong order if I pass it back through opt (or Clang) again.

Could I have missed to duplicate something from BranchInst that might be causing
this? I’d appreciate any help I could get with this.

Sincerely,
Kaarthik A.
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