PR24141: https://llvm.org/bugs/show_bug.cgi?id=24141
contains a test case where we have duplicate entries in a node's uses() list.
After r241826, we use CombineTo() to delete dead nodes when combining the uses into
reciprocal multiplies, but this fails if we encounter the just-deleted node again in
the list.
The solution in this patch is to not add duplicate entries to the list of users that
we will subsequently iterate over. For the test case, this avoids triggering the
combine divisors logic entirely because there really is only one user of the divisor.
Differential Revision: http://reviews.llvm.org/D11345
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243500 91177308-0d34-0410-b5e6-96231b3b80d8
This patch fixes bugs that were exposed by the addition of fast-math-flags in the DAG:
r237046 ( http://reviews.llvm.org/rL237046 ):
1. When replacing a division node, it's not enough to RAUW.
We should call CombineTo() to delete dead nodes and combine again.
2. Because we are changing the DAG, we can't return an empty SDValue
after the transform. As the code comments say:
Visitation implementation - Implement dag node combining for different node types.
The semantics are as follows: Return Value:
SDValue.getNode() == 0 - No change was made
SDValue.getNode() == N - N was replaced, is dead and has been handled.
otherwise - N should be replaced by the returned Operand.
The new test case shows no difference with or without this patch, but it will crash if
we re-apply r237046 or enable FMF via the current -enable-fmf-dag cl::opt.
Differential Revision: http://reviews.llvm.org/D9893
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241826 91177308-0d34-0410-b5e6-96231b3b80d8
Set the transform bar at 2 divisions because the fastest current
x86 FP divider circuit is in SandyBridge / Haswell at 10 cycle
latency (best case) relative to a 5 cycle multiplier.
So that's the worst case for this transform (no latency win),
but multiplies are obviously pipelined while divisions are not,
so there's still a big throughput win which we would expect to
show up in typical FP code.
These are the sequences I'm comparing:
divss %xmm2, %xmm0
mulss %xmm1, %xmm0
divss %xmm2, %xmm0
Becomes:
movss LCPI0_0(%rip), %xmm3 ## xmm3 = mem[0],zero,zero,zero
divss %xmm2, %xmm3
mulss %xmm3, %xmm0
mulss %xmm1, %xmm0
mulss %xmm3, %xmm0
[Ignore for the moment that we don't optimize the chain of 3 multiplies
into 2 independent fmuls followed by 1 dependent fmul...this is the DAG
version of: https://llvm.org/bugs/show_bug.cgi?id=21768 ...if we fix that,
then the transform becomes even more profitable on all targets.]
Differential Revision: http://reviews.llvm.org/D8941
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235012 91177308-0d34-0410-b5e6-96231b3b80d8
