[llvm-commits] [llvm] r141859 - in /llvm/trunk: lib/Target/X86/X86FrameLowering.cpp test/CodeGen/X86/segmented-stacks.ll

[Duncan Sands]

Hi Jakob,

>> More closely follow libgcc, which has code after the `ret' instruction to
>> release the stack segment and reset the stack pointer. Place the code in its own
>> MBB to make the verifier happy.
>
> Wait, what?
>
> Is the verifier happy with a return block with successors? That sounds like a bug ;)
>
> Seriously, though. What is going on here? I am worried that branch folding doesn't understand this, and rearranges the blocks.

Sanjoy sent me the following explanation of why code is added after the "ret":

This is dictated by libgcc.  Here is the relevant comment from
libgcc/config/i386/morestack.S:

# We do a little dance so that the processor's call/return return
# address prediction works out.  The compiler arranges for the caller
# to look like this:
#   call __generic_morestack [ typo in libgcc, should be _morestack]
#   ret
#  L:
#   // carry on with function
# After we allocate more stack, we call L, which is in our caller.
# When that returns (to the predicted instruction), we release the
# stack segment and reset the stack pointer.  We then return to the
# predicted instruction, namely the ret instruction immediately after
# the call to __generic_morestack.  That then returns to the caller of
# the original caller.

Ciao, Duncan.