[x86] unconditional branch not removed in codegen

[lattner]

Bugzilla Link	1200
Resolution	FIXED
Resolved on	Feb 22, 2010 12:42
Version	trunk
OS	All
Attachments	testcase

Extended Description

The attached LLVM code has an uncond branch that isn't removed by branch folding:

_main_bb_2E_i9_2E_i_2E_i932_2E_ce:
subl $28, %esp
movl %esi, 24(%esp)
movl %edi, 20(%esp)
movl 36(%esp), %esi
movl 32(%esp), %eax
jmp LBB1_6 #bb.i9.i.i932.ce
...
LBB1_6: #bb.i9.i.i932.ce
movl (%eax), %edi
movl L_outfile$non_lazy_ptr, %eax
movl (%eax), %eax

The entry block has a single successor, and LBB1_6 has a single predecessor, so branch folding should
merge them.

-Chris

[llvmbot]

By the time branch folding is reached the switch statement has been broken into a binary tree of
compares. The target block of the unconditional branch (LBB1_6) thus has two successors, one of which is
a fallthrough; the branch folder doesn't move the block up in that case:
// If the prior block doesn't fall through into this block, and if this
// block doesn't fall through into some other block, see if we can find a
// place to move this block where a fall-through will happen.
So that's why. The comment doesn't explain why, but obviously hoisting such a block gains no branches
in itself. If repeated enough times, however, you eventually get to a fallthrough that returns, and hoisting
that will gain a branch (in this case).

[llvmbot]

Fixed.
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20070212/044660.html
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20070212/044659.html
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20070212/044661.html
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20070212/044662.html