Remove uses of `forward_as`#3242

SteveBronder · 2025-10-01T20:15:53Z

Summary

Fixes #3190 . forward_as was a workaround for C++14 when we did not have if constexpr and needed both sides of expressions to be able to be evaluated. Now that we have C++17 we can remove all those and cleanup a good bit of the code

Tests

No new tests

Side Effects

Are there any side effects that we should be aware of?

Release notes

Remove use of forward_as in the math library

Checklist

Copyright holder: Steve Bronder

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

…okup picking wrong function

WardBrian · 2025-10-02T14:18:18Z

The couple CI failures look pretty easy to resolve, but I wanted to ask if there was a reason you didn't remove the forward_as code itself in this PR

SteveBronder · 2025-10-02T15:52:36Z

The couple CI failures look pretty easy to resolve, but I wanted to ask if there was a reason you didn't remove the forward_as code itself in this PR

Just backwards compatibility. We don't use it but we don't know if a user has it in their code

WardBrian · 2025-10-02T15:58:59Z

I think it would probably be fine to remove something like that, but if you disagree we should at least mark it is deprecated

SteveBronder · 2025-10-02T16:56:23Z

I'll mark it as depreciated. In general I think when we remove functions that should be a major version bump.

WardBrian

One comment but otherwise looks good!

stan/math/prim/meta/compiler_attributes.hpp

stan-buildbot · 2025-10-02T22:55:03Z

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
arma/arma.stan	0.33	0.38	0.85	-17.35% slower
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.01	0.01	1.03	2.8% faster
gp_regr/gen_gp_data.stan	0.03	0.02	1.11	9.83% faster
gp_regr/gp_regr.stan	0.09	0.09	1.06	5.24% faster
sir/sir.stan	72.42	68.34	1.06	5.62% faster
irt_2pl/irt_2pl.stan	4.36	4.0	1.09	8.25% faster
eight_schools/eight_schools.stan	0.06	0.06	1.0	0.21% faster
pkpd/sim_one_comp_mm_elim_abs.stan	0.28	0.24	1.15	12.68% faster
pkpd/one_comp_mm_elim_abs.stan	19.65	18.45	1.06	6.08% faster
garch/garch.stan	0.44	0.4	1.1	8.95% faster
low_dim_gauss_mix/low_dim_gauss_mix.stan	2.7	2.6	1.04	3.93% faster
arK/arK.stan	1.79	1.73	1.03	3.21% faster
gp_pois_regr/gp_pois_regr.stan	2.76	2.7	1.02	1.99% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	8.66	8.37	1.04	3.41% faster
performance.compilation	177.05	183.08	0.97	-3.4% slower
Mean result: 1.0401275055298054

Jenkins Console Log
Blue Ocean
Commit hash: 2640c757442c9a3273da967ace3feae027b614c6

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

WardBrian · 2025-10-03T13:48:32Z

@SteveBronder looks like there are a couple usages in https://github.com/stan-dev/stan/blob/develop/src/stan/model/indexing/assign_varmat.hpp that will need to be fixed up

WardBrian

🎉

stan-buildbot · 2025-10-04T02:33:17Z

Name	Old Result	New Result	Ratio	Performance change( 1 - new / old )
arma/arma.stan	0.34	0.33	1.05	4.69% faster
low_dim_corr_gauss/low_dim_corr_gauss.stan	0.01	0.01	0.99	-0.79% slower
gp_regr/gen_gp_data.stan	0.03	0.02	1.07	6.78% faster
gp_regr/gp_regr.stan	0.1	0.09	1.04	4.13% faster
sir/sir.stan	74.05	71.98	1.03	2.8% faster
irt_2pl/irt_2pl.stan	4.47	4.23	1.06	5.31% faster
eight_schools/eight_schools.stan	0.06	0.06	1.05	4.78% faster
pkpd/sim_one_comp_mm_elim_abs.stan	0.26	0.25	1.03	2.71% faster
pkpd/one_comp_mm_elim_abs.stan	20.11	19.2	1.05	4.53% faster
garch/garch.stan	0.45	0.41	1.09	8.52% faster
low_dim_gauss_mix/low_dim_gauss_mix.stan	2.8	2.63	1.06	5.92% faster
arK/arK.stan	1.87	1.74	1.08	7.28% faster
gp_pois_regr/gp_pois_regr.stan	3.13	2.72	1.15	13.03% faster
low_dim_gauss_mix_collapse/low_dim_gauss_mix_collapse.stan	9.32	8.54	1.09	8.35% faster
performance.compilation	187.6	190.92	0.98	-1.77% slower
Mean result: 1.0550543545194206

Jenkins Console Log
Blue Ocean
Commit hash: e4209047880771b486dfe532503f536918e27083

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: 3462.680
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 and others added 4 commits October 1, 2025 13:38

update rep_matrix test to decay return type and avoid bug with ADL lo…

a42ab79

…okup picking wrong function

remove all uses for forward_as

734df39

remove all uses for forward_as

b592e63

[Jenkins] auto-formatting by clang-format version 10.0.0-4ubuntu1

0200562

SteveBronder and others added 4 commits October 2, 2025 13:07

remove unused typedefs and mark forward_as deprecated

a66f235

[Jenkins] auto-formatting by clang-format version 10.0.0-4ubuntu1

293eccc

fix cpplint

9ed7532

[Jenkins] auto-formatting by clang-format version 10.0.0-4ubuntu1

f8e7b57

WardBrian reviewed Oct 2, 2025

View reviewed changes

stan/math/prim/meta/compiler_attributes.hpp Show resolved Hide resolved

SteveBronder mentioned this pull request Oct 3, 2025

Removes code that uses forward_as stan-dev/stan#3357

Merged

3 tasks

add empty STAN_DEPRECATED macro when deprecated attribute not available

53a06ec

WardBrian approved these changes Oct 3, 2025

View reviewed changes

SteveBronder merged commit 2f4fe90 into develop Oct 4, 2025
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Remove uses of `forward_as`#3242