[SROA] Teach SROA how to much more intelligently handle split loads and


[SROA] Teach SROA how to much more intelligently handle split loads and

When there are accesses to an entire alloca with an integer
load or store as well as accesses to small pieces of the alloca, SROA
splits up the large integer accesses. In order to do that, it uses bit
math to merge the small accesses into large integers. While this is
effective, it produces insane IR that can cause significant problems in
the rest of the optimizer:

  • It can cause load and store mismatches with GVN on the non-alloca side where we end up loading an i64 (or some such) rather than loading specific elements that are stored.
  • We can't always get rid of the integer bit math, which is why we can't always fix the loads and stores to work well with GVN.
  • This is especially bad when we have operations that mix poorly with integer bit math such as floating point operations.
  • It will block things like the vectorizer which might be able to handle the scalar stores that underly the aggregate.

At the same time, we can't just directly split up these loads and stores
in all cases. If there is actual integer arithmetic involved on the
values, then using integer bit math is actually the perfect lowering
because we can often combine it heavily with the surrounding math.

The solution this patch provides is to find places where SROA is
partitioning aggregates into small elements, and look for splittable
loads and stores that it can split all the way to some other adjacent
load and store. These are uniformly the cases where failing to split the
loads and stores hurts the optimizer that I have seen, and I've looked
extensively at the code produced both from more and less aggressive
approaches to this problem.

However, it is quite tricky to actually do this in SROA. We may have
loads and stores to the same alloca, or other complex patterns that are
hard to handle. This complexity leads to the somewhat subtle algorithm
implemented here. We have to do this entire process as a separate pass
over the partitioning of the alloca, and split up all of the loads prior
to splitting the stores so that we can handle safely the cases of
overlapping, including partially overlapping, loads and stores to the
same alloca. We also have to reconstitute the post-split slice
configuration so we can avoid iterating again over all the alloca uses
(the slow part of SROA). But we also have to ensure that when we split
up loads and stores to *other* allocas, we *do* re-iterate over them in
SROA to adapt to the more refined partitioning now required.

With this, I actually think we can fix a long-standing TODO in SROA
where I avoided splitting as many loads and stores as probably should be
splittable. This limitation historically mitigated the fallout of all
the bad things mentioned above. Now that we have more intelligent
handling, I plan to remove the FIXME and more aggressively mark integer
loads and stores as splittable. I'll do that in a follow-up patch to
help with bisecting any fallout.

The net result of this change should be more fine-grained and accurate
scalars being formed out of aggregates. At the very least, Clang now
generates perfect code for this high-level test case using

#include <complex>

void g1(std::complex<float> &x, float a, float b) {
  x += std::complex<float>(a, b);
void g2(std::complex<float> &x, float a, float b) {
  x -= std::complex<float>(a, b);

void foo(const std::complex<float> &x, float a, float b,
         std::complex<float> &x1, std::complex<float> &x2) {
  std::complex<float> l1 = x;
  g1(l1, a, b);
  std::complex<float> l2 = x;
  g2(l2, a, b);
  x1 = l1;
  x2 = l2;

This code isn't just hypothetical either. It was reduced out of the hot
inner loops of essentially every part of the Eigen math library when
using std::complex<float>. Those loops would consistently and
pervasively hop between the floating point unit and the integer unit due
to bit math extraction and insertion of floating point values that were
"stored" in a 64-bit integer register around the loop backedge.

So far, this change has passed a bootstrap and I have done some other
testing and so far, no issues. That doesn't mean there won't be though,
so I'll be prepared to help with any fallout. If you performance swings
in particular, please let me know. I'm very curious what all the impact
of this change will be. Stay tuned for the follow-up to also split more
integer loads and stores.


chandlercJan 1 2015, 3:54 AM
rL225060: CodeGen: Don't crash when a lambda uses a local constexpr variable