SimplifyCFG: fix an infinite jump-threading loop.

[eeckstein]

The JumpThreadingCost map in Simplify CFG is used to prevent infinite jump threading loops.
There was a missing update of the cost for blocks which are cloned:
Jump threading loops were prevented for infinitely cloning the original block, but not for re-cloning the cloned block.

A test case is already added in #35604

rdar://73357726, [SR-14068]

[eeckstein]

@swift-ci test

[eeckstein]

@swift-ci benchmark

[eeckstein]

@swift-ci benchmark

[eeckstein]

@swift-ci test

[swift-ci]

Performance: -O

Regression	OLD	NEW	DELTA	RATIO
FlattenListLoop	1628	2513	+54.4%	0.65x (?)
Data.append.Sequence.809B.Count.I	91	99	+8.8%	0.92x (?)
StringBuilderLong	1360	1470	+8.1%	0.93x (?)
UTF8Decode_InitFromBytes_ascii_as_ascii	497	537	+8.0%	0.93x (?)
Improvement	OLD	NEW	DELTA	RATIO
FlattenListFlatMap	5395	4192	-22.3%	1.29x (?)

Code size: -O

Performance: -Osize

Regression	OLD	NEW	DELTA	RATIO
FlattenListLoop	1630	2504	+53.6%	0.65x (?)
FlattenListFlatMap	5552	6586	+18.6%	0.84x (?)

Code size: -Osize

Performance: -Onone

Regression	OLD	NEW	DELTA	RATIO
UTF8Decode_InitFromData_ascii_as_ascii	723	785	+8.6%	0.92x (?)

Code size: -swiftlibs

How to read the data

The tables contain differences in performance which are larger than 8% and differences in code size which are larger than 1%.

If you see any unexpected regressions, you should consider fixing the
regressions before you merge the PR.

Noise: Sometimes the performance results (not code size!) contain false
alarms. Unexpected regressions which are marked with '(?)' are probably noise.
If you see regressions which you cannot explain you can try to run the
benchmarks again. If regressions still show up, please consult with the
performance team (@eeckstein).

Hardware Overview

  Model Name: Mac Pro
  Model Identifier: MacPro6,1
  Processor Name: 12-Core Intel Xeon E5
  Processor Speed: 2.7 GHz
  Number of Processors: 1
  Total Number of Cores: 12
  L2 Cache (per Core): 256 KB
  L3 Cache: 30 MB
  Memory: 64 GB

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - fe928d5

[eeckstein]

@swift-ci test macOS

[eeckstein]

@swift-ci test macOS

[atrick]

It looks like you want to use JumpThreadingCost to be an accumulator on any block that has been cloned so that the current cost is the original cost multiplied by the number of clones. That's not obvious from the comments.

I do think it's a good idea to generally limit run-away jump-threading. But I'm still worried about small loops, so still like this safeguard: #35608

The thing that is particularly baffling to me is that this cost is initialized in simplifyBranchBlock which is unreliable.

[atrick]

This looks good to me. I'll merge this PR and add the necessary comments in a follow-up PR.