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120fce5540
or when splitting loops with a common header into multiple loops. In particular
the old code would always insert the preheader before the old loop header. This
is disasterous in cases where the loop hasn't been rotated. For example, it can
produce code like:
.. outside the loop...
jmp LBB1_2 #bb13.outer
LBB1_1: #bb1
movsd 8(%esp,%esi,8), %xmm1
mulsd (%edi), %xmm1
addsd %xmm0, %xmm1
addl $24, %edi
incl %esi
jmp LBB1_3 #bb13
LBB1_2: #bb13.outer
leal (%edx,%eax,8), %edi
pxor %xmm1, %xmm1
xorl %esi, %esi
LBB1_3: #bb13
movapd %xmm1, %xmm0
cmpl $4, %esi
jl LBB1_1 #bb1
Note that the loop body is actually LBB1_1 + LBB1_3, which means that the
loop now contains an uncond branch WITHIN it to jump around the inserted
loop header (LBB1_2). Doh.
This patch changes the preheader insertion code to insert it in the right
spot, producing this code:
... outside the loop, fall into the header ...
LBB1_1: #bb13.outer
leal (%edx,%eax,8), %esi
pxor %xmm0, %xmm0
xorl %edi, %edi
jmp LBB1_3 #bb13
LBB1_2: #bb1
movsd 8(%esp,%edi,8), %xmm0
mulsd (%esi), %xmm0
addsd %xmm1, %xmm0
addl $24, %esi
incl %edi
LBB1_3: #bb13
movapd %xmm0, %xmm1
cmpl $4, %edi
jl LBB1_2 #bb1
Totally crazy, no branch in the loop! :)
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@30587 91177308-0d34-0410-b5e6-96231b3b80d8
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