Use if constexpr almost everywhere and cleanup type traits

[SteveBronder]

Summary

This PR does two main things which are mostly just a series of clever greps

1. Replaces almost all places we use if with only constexpr evaluated conditionals with if constexpr
2. Change the probability distributions to use is_autodiff_v instead of !is_constant<>::value

I think is_any_autodiff is way easier to read then !is_all_constant

Tests

No new tests

Side Effects

Previously is_autodiff only passed for scalars. But now we have an is_autodiff_scalar that only checks if the type is a scalar fvar or var and is_autodiff will return a value of true for any type whose scalar_type is an autodiff type. This should slightly decrease compile times since now the compiler knows it does not need to go through certain paths and so will never generate anything for the paths we do not go down.

Release notes

Replace standard if statements with if constexpr where posssible.

Checklist

• Copyright holder: (fill in copyright holder information)

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• the basic tests are passing

  • unit tests pass (to run, use: ./runTests.py test/unit)
  • header checks pass, (make test-headers)
  • dependencies checks pass, (make test-math-dependencies)
  • docs build, (make doxygen)
  • code passes the built in C++ standards checks (make cpplint)

• the code is written in idiomatic C++ and changes are documented in the doxygen

• the new changes are tested

[syclik]

@SteveBronder: amazing! It is much easier to read with your changes.

I added a couple of comments. I think the only real one to answer is what's up with the compiler flags? Was that intentional?

And it didn't pass some of the CI tests. Once that's all good, I think we merge!

[syclik]

nice find!

[SteveBronder]

@syclik thanks!

And it didn't pass some of the CI tests. Once that's all good, I think we merge!
Yes upstream was failing because I removed the actual is_autodiff type trait and had just a is_autodiff_v helper. Pushing that now so should be good.

This is all happening so that we can finally do #3190 and get rid of one of the odder template tricks we had to do in C++14

[stan-buildbot]

---

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
arma/arma.stan	0.33	0.33	1.01	0.6% faster
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.01	0.01	1.04	4.03% faster
gp_regr/gen_gp_data.stan	0.03	0.02	1.11	9.99% faster
gp_regr/gp_regr.stan	0.1	0.09	1.07	6.24% faster
sir/sir.stan	72.86	68.44	1.06	6.07% faster
irt_2pl/irt_2pl.stan	4.25	3.89	1.09	8.3% faster
eight_schools/eight_schools.stan	0.06	0.05	1.11	10.17% faster
pkpd/sim_one_comp_mm_elim_abs.stan	0.26	0.24	1.09	8.48% faster
pkpd/one_comp_mm_elim_abs.stan	19.7	18.26	1.08	7.32% faster
garch/garch.stan	0.45	0.4	1.12	10.63% faster
low_dim_gauss_mix/low_dim_gauss_mix.stan	2.88	2.54	1.13	11.82% faster
arK/arK.stan	1.82	1.7	1.07	6.83% faster
gp_pois_regr/gp_pois_regr.stan	2.89	2.71	1.07	6.14% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.92	8.37	1.07	6.19% faster
performance.compilation	178.56	176.3	1.01	1.27% faster
Mean result: 1.0757386017111612

---

Jenkins Console Log
Blue Ocean
Commit hash: ab0b7021ea9509b9bf50dd771c7d9c38dcac690c

---

Machine information

No LSB modules are available. Distributor ID: Ubuntu Description: Ubuntu 20.04.3 LTS Release: 20.04 Codename: focal

CPU:
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
Address sizes: 46 bits physical, 48 bits virtual
CPU(s): 80
On-line CPU(s) list: 0-79
Thread(s) per core: 2
Core(s) per socket: 20
Socket(s): 2
NUMA node(s): 2
Vendor ID: GenuineIntel
CPU family: 6
Model: 85
Model name: Intel(R) Xeon(R) Gold 6148 CPU @ 2.40GHz
Stepping: 4
CPU MHz: 2400.000
CPU max MHz: 3700.0000
CPU min MHz: 1000.0000
BogoMIPS: 4800.00
Virtualization: VT-x
L1d cache: 1.3 MiB
L1i cache: 1.3 MiB
L2 cache: 40 MiB
L3 cache: 55 MiB
NUMA node0 CPU(s): 0,2,4,6,8,10,12,14,16,18,20,22,24,26,28,30,32,34,36,38,40,42,44,46,48,50,52,54,56,58,60,62,64,66,68,70,72,74,76,78
NUMA node1 CPU(s): 1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31,33,35,37,39,41,43,45,47,49,51,53,55,57,59,61,63,65,67,69,71,73,75,77,79
Vulnerability Gather data sampling: Mitigation; Microcode
Vulnerability Itlb multihit: KVM: Mitigation: VMX disabled
Vulnerability L1tf: Mitigation; PTE Inversion; VMX conditional cache flushes, SMT vulnerable
Vulnerability Mds: Mitigation; Clear CPU buffers; SMT vulnerable
Vulnerability Meltdown: Mitigation; PTI
Vulnerability Mmio stale data: Mitigation; Clear CPU buffers; SMT vulnerable
Vulnerability Reg file data sampling: Not affected
Vulnerability Retbleed: Mitigation; IBRS
Vulnerability Spec rstack overflow: Not affected
Vulnerability Spec store bypass: Mitigation; Speculative Store Bypass disabled via prctl
Vulnerability Spectre v1: Mitigation; usercopy/swapgs barriers and __user pointer sanitization
Vulnerability Spectre v2: Mitigation; IBRS; IBPB conditional; STIBP conditional; RSB filling; PBRSB-eIBRS Not affected; BHI Not affected
Vulnerability Srbds: Not affected
Vulnerability Tsx async abort: Mitigation; Clear CPU buffers; SMT vulnerable
Flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb cat_l3 cdp_l3 invpcid_single pti intel_ppin ssbd mba ibrs ibpb stibp tpr_shadow vnmi flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm cqm mpx rdt_a avx512f avx512dq rdseed adx smap clflushopt clwb intel_pt avx512cd avx512bw avx512vl xsaveopt xsavec xgetbv1 xsaves cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local dtherm ida arat pln pts hwp hwp_act_window hwp_epp hwp_pkg_req pku ospke md_clear flush_l1d arch_capabilities

G++:
g++ (Ubuntu 9.4.0-1ubuntu1~20.04) 9.4.0
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Clang:
clang version 10.0.0-4ubuntu1
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

[SteveBronder]

@WardBrian @syclik good to merge?

[syclik]

LGTM!

[syclik]

@SteveBronder, it's good to go!