This reverts commit bd7949bcd86633bd4203b2ba6f891aea00fce4d1.
Revert this patch since reviwers have different opinions regarding
the approach in post-commit review.
Will open RFC for further discussion.
Differential Revision: https://reviews.llvm.org/D132408
SimplifyCFG folds
bool foo() {
if (cond1) return false;
if (cond2) return false;
return true;
}
as
bool foo() {
if (cond1 | cond2) return false
return true;
}
'cond2' is called 'bonus insts' in branch folding since they introduce overhead
since the original CFG could do early exit but the folded CFG always executes
them. SimplifyCFG calculates the costs of 'bonus insts' of a folding a BB into
its predecessor BB which shares the destination. If it is below bonus-inst-threshold,
SimplifyCFG will fold that BB into its predecessor and cond2 will always be executed.
When SimplifyCFG calculates the cost of 'bonus insts', it only consider 'bonus' insts
in the current BB to be considered for folding. This causes issue for unrolled loops
which share destinations, e.g.
bool foo(int *a) {
for (int i = 0; i < 32; i++)
if (a[i] > 0) return false;
return true;
}
After unrolling, it becomes
bool foo(int *a) {
if(a[0]>0) return false
if(a[1]>0) return false;
//...
if(a[31]>0) return false;
return true;
}
SimplifyCFG will merge each BB with its predecessor BB,
and ends up with 32 'bonus insts' which are always executed, which
is much slower than the original CFG.
The root cause is that SimplifyCFG does not consider the
accumulated cost of 'bonus insts' which are folded from
different BB's.
This patch fixes that by introducing a ValueMap to track
costs of 'bonus insts' coming from different BB's into
the same BB, and cuts off if the accumulated cost
exceeds a threshold.
Reviewed by: Artem Belevich, Florian Hahn, Nikita Popov, Matt Arsenault
Differential Revision: https://reviews.llvm.org/D132408
I can't seem to wrap my head around the proper fix here,
we should be fine without this requirement, iff we can form this form,
but the naive attempt (https://reviews.llvm.org/D106317) has failed.
So just to unblock the release, put up a restriction.
Fixes https://bugs.llvm.org/show_bug.cgi?id=51125
There was an alias between 'simplifycfg' and 'simplify-cfg' in the
PassRegistry. That was the original reason for this patch, which
effectively removes the alias.
This patch also replaces all occurrances of 'simplify-cfg'
by 'simplifycfg'. Reason for choosing that form for the name is
that it matches the DEBUG_TYPE for the pass, and the legacy PM name
and also how it is spelled out in other passes such as
'loop-simplifycfg', and in other options such as
'simplifycfg-merge-cond-stores'.
I for some reason the name should be changed to 'simplify-cfg' in
the future, then I think such a renaming should be more widely done
and not only impacting the PassRegistry.
Reviewed By: aeubanks
Differential Revision: https://reviews.llvm.org/D105627
I have previously tried doing that in
b33fbbaa34f0fe9fb16789afc72ae424c1825b69 / d38205144febf4dc42c9270c6aa3d978f1ef65e1,
but eventually it was pointed out that the approach taken there
was just broken wrt how the uses of bonus instructions are updated
to account for the fact that they should now use either bonus instruction
or the cloned bonus instruction. In particluar, all that manual handling
of PHI nodes in successors was just wrong.
But, the fix is actually much much simpler than my initial approach:
just tell SSAUpdate about both instances of bonus instruction,
and let it deal with all the PHI handling.
Alive2 confirms that the reproducers from the original bugs (@pr48450*)
are now handled correctly.
This effectively reverts commit 59560e85897afc50090b6c3d920bacfd28b49d06,
effectively relanding b33fbbaa34f0fe9fb16789afc72ae424c1825b69.
Even though d38205144febf4dc42c9270c6aa3d978f1ef65e1 was mostly a correct
fix for the external non-PHI users, it's not a *generally* correct fix,
because the 'placeholder' values in those trivial PHI's we create
shouldn't be *always* 'undef', but the PHI itself for the backedges,
else we end up with wrong value, as the `@pr48450_2` test shows.
But we can't just do that, because we can't check that the PHI
can be it's own incoming value when coming from certain predecessor,
because we don't have a dominator tree.
So until we can address this correctness problem properly,
ensure that we don't perform the transformation
if there are such problematic external uses.
Making dominator tree available there is going to be involved,
since `-simplifycfg` pass currently does not preserve/update domtree...
This was orginally committed in 2245fb8aaa1c1f85f53f7b19a1ee3ac69b1a1dfe.
but was immediately reverted in f3abd54958ab90ba7c100d3fa936a3ce0dd2ad04
because of a PHI handling issue.
Original commit message:
1. It doesn't make sense to enforce that the bonus instruction
is only used once in it's basic block. What matters is
whether those user instructions fit within our budget, sure,
but that is another question.
2. It doesn't make sense to enforce that said bonus instructions
are only used within their basic block. Perhaps the branch
condition isn't using the value computed by said bonus instruction,
and said bonus instruction is simply being calculated
to be used in successors?
So iff we can clone bonus instructions, to lift these restrictions,
we just need to carefully update their external uses
to use the new cloned instructions.
Notably, this transform (even without this change) appears to be
poison-unsafe as per alive2, but is otherwise (including the patch) legal.
We don't introduce any new PHI nodes, but only "move" the instructions
around, i'm not really seeing much potential for extra cost modelling
for the transform, especially since now we allow at most one such
bonus instruction by default.
This causes the fold to fire +11.4% more (13216 -> 14725)
as of vanilla llvm test-suite + RawSpeed.
The motivational pattern is IEEE-754-2008 Binary16->Binary32
extension code:
ca57d77fb2/src/librawspeed/common/FloatingPoint.h (L115-L120)
^ that should be a switch, but it is not now: https://godbolt.org/z/bvja5v
That being said, even thought this seemed like this would fix it: https://godbolt.org/z/xGq3TM
apparently that fold is happening somewhere else afterall,
so something else also has a similar 'artificial' restriction.
Many bots are unhappy, at the very least missed a few codegen tests,
and possibly this has a logic hole inducing a miscompile
(will be really awesome to have ready reproducer..)
Need to investigate.
This reverts commit 2245fb8aaa1c1f85f53f7b19a1ee3ac69b1a1dfe.
1. It doesn't make sense to enforce that the bonus instruction
is only used once in it's basic block. What matters is
whether those user instructions fit within our budget, sure,
but that is another question.
2. It doesn't make sense to enforce that said bonus instructions
are only used within their basic block. Perhaps the branch
condition isn't using the value computed by said bonus instruction,
and said bonus instruction is simply being calculated
to be used in successors?
So iff we can clone bonus instructions, to lift these restrictions,
we just need to carefully update their external uses
to use the new cloned instructions.
Notably, this transform (even without this change) appears to be
poison-unsafe as per alive2, but is otherwise (including the patch) legal.
We don't introduce any new PHI nodes, but only "move" the instructions
around, i'm not really seeing much potential for extra cost modelling
for the transform, especially since now we allow at most one such
bonus instruction by default.
This causes the fold to fire +11.4% more (13216 -> 14725)
as of vanilla llvm test-suite + RawSpeed.
The motivational pattern is IEEE-754-2008 Binary16->Binary32
extension code:
ca57d77fb2/src/librawspeed/common/FloatingPoint.h (L115-L120)
^ that should be a switch, but it is not now: https://godbolt.org/z/bvja5v
That being said, even thought this seemed like this would fix it: https://godbolt.org/z/xGq3TM
apparently that fold is happening somewhere else afterall,
so something else also has a similar 'artificial' restriction.
The legacy pass is called "loop-unroll", but in the new PM it's called "unroll".
Also applied to unroll-and-jam and unroll-full.
Fixes various check-llvm tests when NPM is turned on.
Reviewed By: Whitney, dmgreen
Differential Revision: https://reviews.llvm.org/D82590
