Miscompilation using `-C target-cpu=znver4` in `"Rust 1.95"..="nightly"`

[IceTDrinker]

It looks like a maths heavy loop is getting miscompiled when using the znver4 target (or znver5) znver3 is ok. function of interest is convert_add_backward_torus_non_split and is marked inline(never) for debug.

First minimization step (from larger bundle of code causing issue) is on this commit in the TFHE-rs project https://github.com/zama-ai/tfhe-rs/blob/33b1d447e17794090cfd8644c93dd110af8dc9c3/tfhe/examples/miscompile_znver4_1_95_minified.rs

main function causing problems for now : https://github.com/zama-ai/tfhe-rs/blob/33b1d447e17794090cfd8644c93dd110af8dc9c3/tfhe/src/core_crypto/fft_impl/fft128/math/fft/mod.rs#L277 (depends on other parts of the code, not yet minified).

command to run:

RUSTFLAGS="-C target-cpu=znver4" cargo +1.96.1 run --profile=release --example miscompile_znver4_1_95_minified -p tfhe

The bug does not happen on 1.94, it happens in 1.95 and 1.96.1, (yes it was just released and I was eager to check whether the MIR miscompilation fix would help me, it does not).
The bug happens when using RUSTFLAGS="-C target-cpu=znver4" or znver5

Given the above, it seems it's a problem with the LLVM update in 1.95.

It's not a complete minimization but the asm of the problematic function is down to ~600 lines which seemed like a first good stop to share what I found since I'm never quite sure which tools to run and tracking down this function already took some time to investigate (and you guys always seem to have a magic command that solves the remaining step 🥲 ).

ASM generated by cargo-show-asm with RUSTFLAGS="-C target-cpu=znver4" cargo +1.96.1 asm --profile=release --example miscompile_znver4_1_95_minified -p tfhe convert_add_backward_torus_non_split

1_94_convert_fn.asm.txt
1_95_convert_fn.asm.txt
1_96_1_convert_fn.asm.txt

edit: note 1_95 and 1_96_1 seem to generate the same ASM, 1_94 gives the expected result not the others, nightly-2026-06-30 also affected by the bug

More than happy to run some specific commands to isolate the culprit, if it needs further minimization, it will be for tomorrow.

Cheers

Meta

rustc --version --verbose:

rustc --version --verbose
rustc 1.96.1 (31fca3adb 2026-06-26)
binary: rustc
commit-hash: 31fca3adb283cc9dfd56b49cdee9a96eb9c96ffd
commit-date: 2026-06-26
host: x86_64-unknown-linux-gnu
release: 1.96.1
LLVM version: 22.1.2

Backtrace

thread 'main' (1684993) panicked at tfhe/examples/miscompile_znver4_1_95_minified.rs:424:5:
assertion `left == right` failed
  left: [110628347648513656525279463143647477760, 323343687258746376232127866058041196544, 168368331461313543236514255564561186816, 223572064150608245109958874458503512064, 200998675409509307462117392191112347648, 104262090599876955312522374857204170752, 206366409059269844144294612422595969024, 115908057266915935870361873832183070720, 294474291842428489851353029051649359872, 338462349666572665722512470713405800448, 156479399767262362804223335807735300096, 195514418804515072368547681055851675648, 54628167523137871920003290654109073408, 155720349294498777208265221001160163328, 311787847511774969981920820476642328576, 60231124039915007296739898768782000128, 185767607058272603416941301530544308224, 170063967125022796368108148411605188608, 56500371446833321559927327838514970624, 244468389644458908745320156377013813248, 137422720908329091455417294420713668608, 340046997699111938715009026493295624192, 197790070082303212996963697954410987520, 13668427859988941398144479426475196416, 291219980515134076034946654229982347264, 12701926893200253598754838068775092224, 282014549667405799324834199364133978112, 269679919422378724576615530411275059200, 242614004738315548715549568811048894464, 52369944764241406066784668607047532544, 203444276651113211647433733708049285120, 221236464140282991909083243405161529344, 325641853132440950746742596394352115712, 25352960880781969159691897798161072128, 288284445005831430756877138984486043648, 63575455532239796994241148217280430080, 310160547646534023416910464634889175040, 81850016977363618150233197361556881408, 222654181033355780307440649353209839616, 338181537169360186101097726145659404288, 271658560388814081565462735990753853440, 294689778987957421051799398248383250432, 51320199416502490745341719105524203520, 274122467151084297271999050513494573056, 238692735073840025916346887458024062976, 336797072449945162129215248348592734208, 293173370339098568261516463216147824640, 258752964913388711550842250648652611584, 202109684818301460388101100513603354624, 235283819743933443829201166479197732864, 62672175672163279096869928700800401408, 59883541104698390807348562072508039168, 171035195046367844865705098687241256960, 256281031527711753244362820201857679360, 86223756836905577201991902596612227072, 100836221050396381570624882030445330432, 84504883282399559380472496302181056512, 147892420574445361387455953629211525120, 209501301434016272795799112612883464192, 165765530677617651003780154914288697344, 34521668191431499691658167823934423040, 74246931156132331463799390541338116096, 163326699965306187723320992433522606080, 139335700096923906417888747545102909440, 114479001074612009076550637919752159232, 280840936652226014940419762556363603968, 24849963284491906354184718085443813376, 186345231968282196621130514410265116672, 337744886571209283103231385930523738112, 37141958216579566587184006915539599360, 35817992431563218770916045112989450240, 64527500905137802788555732239370944512, 339836411439508943978361330175689359360, 309600974781582287249430319586485469184, 334526238982730382585032824552769454080, 29585545155263405437456580917657075712, 175856000025518674130482555494732922880, 148849551523140603213369251698250350592, 178773738650851006954597892077452787712, 295978754564159369035590923589099978752, 85651346277357826424046403107535454208, 281980581032952012638555222823757938688, 314747114562634199261725301305251987456, 265474551396449756103735207279942696960, 162704625829111309089242894121937403904, 335016513163925389169259835877073354752, 136792310554488946359912536364581650432, 333165697502571922138234331236726210560, 225527248581880826125817928001209237504, 44453307910826272041184872326351552512, 275421295703642764097032283484934111232, 143203694334365762314524421413664718848, 314721952503697740466688666406035128320, 134479020149580521523610863441000005632, 240963426437665007323591590302707613696, 170416467900191875253337953067151130624, 125089315810126123976903084759396122624, 192381644730268999980710714963212107776, 321115488100634198296732923288096866304, 111458884557427924383547602350385397760, 235416871209646964530867125067941150720, 134980517862417214196329097196344442880, 165707873376394959485985092557021904896, 271006246657083606174765267648321159168, 218648985333776416376716485543736639488, 210021974178542763838457156618992746496, 5381483225407811962991550104678170624, 45864351687382535027463722063443787776, 89778931297518165081536567480583979008, 81409068043157426122853240694357622784, 113711360490918478622591219592283029504, 81838055172799648321357355755073699840, 241476941619763625078236613757284384768, 332400432915610472237059421202034982912, 109421294795971486767924473066894131200, 122479388409745588471079652764787146752, 156962613531438305312109480742853017600, 178757133837192791550557366508666748928, 33953664745999860546209956893583998976, 23375661297600291534468116856575098880, 43151083359644055831449146744836718592, 162651657928906068420048020282147864576, 39697886681006200553278732086275997696, 4782669351537038341979624611858350080, 18711862779008672585519836597520957440, 5540085937468659940368234632680833024, 335740470725590627380127612221570154496, 124975272598754403074276038647713103872]
 right: [110628347648513656525279463143647477760, 323343687258746376232127866058041196544, 168368331461313543236514255564561186816, 223572064150608245109958874458503512064, 200998675409509307462117392191112347648, 104262090599876955312522374857204170752, 206366409059269844144294612422595969024, 115908057266915935870361873832183070720, 294474291842428489851353029051649359872, 338462349666572665722512470713405800448, 156479399767262349330681607513576570880, 195514418804515072368547681055851675648, 54628167523137871920003290654109073408, 155720349294498777208265221001160163328, 311787847511774969981920820476642328576, 60231124039915007296739898768782000128, 185767607058272603416941301530544308224, 170063967125022796368108148411605188608, 56500371446833321559927327838514970624, 244468389644458908745320156377013813248, 137422720908329091455417294420713668608, 340046997699111938715009026493295624192, 197790070082303210780606368911638659072, 13668427859988941398144479426475196416, 291219980515134076034946654229982347264, 12701926893200253598754838068775092224, 282014549667405799324834199364133978112, 269679919422378724576615530411275059200, 242614004738315548715549568811048894464, 52369944764241406066784668607047532544, 203444276651113176167708262963697680384, 221236464140282991909083243405161529344, 325641853132440950746742596394352115712, 25352960880781969159691897798161072128, 288284445005831430756877138984486043648, 63575455532239796994241148217280430080, 310160547646534023416910464634889175040, 81850016977363618150233197361556881408, 222654181033355780307440649353209839616, 338181537169360186101097726145659404288, 271658560388814081565462735990753853440, 294689778987957421051799398248383250432, 51320199416502490745341719105524203520, 274122467151084297271999050513494573056, 238692735073840025916346887458024062976, 336797072449945162129215248348592734208, 293173370339098568261516463216147824640, 258752964913388711550842250648652611584, 202109684818301460388101100513603354624, 235283819743933443829201166479197732864, 62672175672163279096869928700800401408, 59883541104698390807348562072508039168, 171035195046367844865705098687241256960, 256281031527711753244362820201857679360, 86223756836905567715917229284200022016, 100836221050396381570624882030445330432, 84504883282399559380472496302181056512, 147892420574445361387455953629211525120, 209501301434016240725956085333593423872, 165765530677617651003780154914288697344, 34521668191431499691658167823934423040, 74246931156132331463799390541338116096, 163326699965306187723320992433522606080, 139335700096923906417888747545102909440, 114479001074612009076550637919752159232, 280840936652226014940419762556363603968, 24849963284491904167930293376946012160, 186345231968282196621130514410265116672, 337744886571209283103231385930523738112, 37141958216579566587184006915539599360, 35817992431563217277637954154872700928, 64527500905137802788555732239370944512, 339836411439508943978361330175689359360, 309600974781582287249430319586485469184, 334526238982730382585032824552769454080, 29585545155263405437456580917657075712, 175856000025518674130482555494732922880, 148849551523140603213369251698250350592, 178773738650851000262783507523157622784, 295978754564159369035590923589099978752, 85651346277357826424046403107535454208, 281980581032952012638555222823757938688, 314747114562634161866017185908765229056, 265474551396449756103735207279942696960, 162704625829111309089242894121937403904, 335016513163925389169259835877073354752, 136792310554488938638589632217203343360, 333165697502571922138234331236726210560, 225527248581880826125817928001209237504, 44453307910826272041184872326351552512, 275421295703642764097032283484934111232, 143203694334365762314524421413664718848, 314721952503697740466688666406035128320, 134479020149580521523610863441000005632, 240963426437664988438278118418050711552, 170416467900191875253337953067151130624, 125089315810126123976903084759396122624, 192381644730268999980710714963212107776, 321115488100634191080141799441523802112, 111458884557427924383547602350385397760, 235416871209646964530867125067941150720, 134980517862417214196329097196344442880, 165707873376394959485985092557021904896, 271006246657083606174765267648321159168, 218648985333776416376716485543736639488, 210021974178542763838457156618992746496, 5381483225407811366211259970000781312, 45864351687382535027463722063443787776, 89778931297518165081536567480583979008, 81409068043157426122853240694357622784, 113711360490918471035611818229273460736, 81838055172799648321357355755073699840, 241476941619763625078236613757284384768, 332400432915610472237059421202034982912, 109421294795971476064876834609776558080, 122479388409745588471079652764787146752, 156962613531438305312109480742853017600, 178757133837192791550557366508666748928, 33953664745999860546209956893583998976, 23375661297600291534468116856575098880, 43151083359644055831449146744836718592, 162651657928906068420048020282147864576, 39697886681006198137333900613697667072, 4782669351537038341979624611858350080, 18711862779008672585519836597520957440, 5540085937468659940368234632680833024, 335740470725590627380127612221570154496, 124975272598754403074276038647713103872]
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/31fca3adb283cc9dfd56b49cdee9a96eb9c96ffd/library/std/src/panicking.rs:689:5
   1: core::panicking::panic_fmt
             at /rustc/31fca3adb283cc9dfd56b49cdee9a96eb9c96ffd/library/core/src/panicking.rs:80:14
   2: core::panicking::assert_failed_inner
             at /rustc/31fca3adb283cc9dfd56b49cdee9a96eb9c96ffd/library/core/src/panicking.rs:439:17
   3: core::panicking::assert_failed
             at /rustc/31fca3adb283cc9dfd56b49cdee9a96eb9c96ffd/library/core/src/panicking.rs:394:5
   4: miscompile_znver4_1_95_minified::main
             at ./tfhe/examples/miscompile_znver4_1_95_minified.rs:424:5
   5: core::ops::function::FnOnce::call_once
             at /rustc/31fca3adb283cc9dfd56b49cdee9a96eb9c96ffd/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace

[workingjubilee]

does this still affect nightly?

[IceTDrinker]

@workingjubilee checking now

[IceTDrinker]

Yes it does

rustc +nightly --version --verbose
rustc 1.98.0-nightly (0966944 2026-06-29)
binary: rustc
commit-hash: 0966944
commit-date: 2026-06-29
host: x86_64-unknown-linux-gnu
release: 1.98.0-nightly
LLVM version: 22.1.8

[workingjubilee]

Thanks for confirming that!

[workingjubilee]

@IceTDrinker Can you test if the miscompilation occurs if you enable most or all of the features that are in znver4, without specifying -Ctarget-cpu? That involves at least -Ctarget-feature=+avx512vbmi2,+avx512vl, and probably other features as well.

The list is here: https://github.com/llvm/llvm-project/blob/1805f5f7d1fd8ac4f5225f8b87124f59d82759f5/llvm/lib/Target/X86/X86.td#L1707-L1720