[libc] Fix Off By One Errors In Printf Long Double

[michaelrj-google]

Two major off-by-one errors are fixed in this patch. The first is in
float_to_string.h with length_for_num, which wasn't accounting for the
implicit leading bit when calculating the length of a number, causing
a missing digit on 80 bit float max. The other off-by-one is the
ryu_long_double_constants.h (a.k.a the Mega Table) not having any
entries for the last POW10_OFFSET in POW10_SPLIT. This was also found on
80 bit float max. Finally, the integer calculation mode was using a
slightly too short integer, again on 80 bit float max, not accounting
for the mantissa width. All of these are fixed in this patch.

[llvmbot]

@llvm/pr-subscribers-libc

Changes

Two major off-by-one errors are fixed in this patch. The first is in
float_to_string.h with length_for_num, which wasn't accounting for the
implicit leading bit when calculating the length of a number, causing
a missing digit on 80 bit float max. The other off-by-one is the
ryu_long_double_constants.h (a.k.a the Mega Table) not having any
entries for the last POW10_OFFSET in POW10_SPLIT. This was also found on
80 bit float max. Finally, the integer calculation mode was using a
slightly too short integer, again on 80 bit float max, not accounting
for the mantissa width. All of these are fixed in this patch.

---

Patch is 66.76 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/66957.diff

4 Files Affected:

• (modified) libc/src/__support/float_to_string.h (+5-3)
• (modified) libc/src/__support/ryu_long_double_constants.h (+552)
• (modified) libc/test/src/stdio/sprintf_test.cpp (+79)
• (modified) libc/utils/mathtools/ryu_tablegen.py (+2-2)

diff --git a/libc/src/__support/float_to_string.h b/libc/src/__support/float_to_string.h
index 4714068eadf0e52..4c58651dc6884b7 100644
--- a/libc/src/__support/float_to_string.h
+++ b/libc/src/__support/float_to_string.h
@@ -157,7 +157,7 @@ LIBC_INLINE constexpr uint32_t ceil_log10_pow2(const uint32_t e) {
 LIBC_INLINE constexpr uint32_t length_for_num(const uint32_t idx,
                                               const uint32_t mantissa_width) {
   //+8 to round up when dividing by 9
-  return (ceil_log10_pow2(idx) + ceil_log10_pow2(mantissa_width) +
+  return (ceil_log10_pow2(idx) + ceil_log10_pow2(mantissa_width + 1) +
           (BLOCK_SIZE - 1)) /
          BLOCK_SIZE;
   // return (ceil_log10_pow2(16 * idx + mantissa_width) + 8) / 9;
@@ -666,7 +666,7 @@ FloatToString<long double>::get_positive_block(int block_index) {
     // ----------------------------- INT CALC MODE -----------------------------
     const int32_t SHIFT_CONST = CALC_SHIFT_CONST;
     const uint64_t MAX_POW_2_SIZE =
-        exponent + CALC_SHIFT_CONST - (BLOCK_SIZE * block_index);
+        pos_exp + CALC_SHIFT_CONST - (BLOCK_SIZE * block_index);
     const uint64_t MAX_POW_5_SIZE =
         internal::log2_pow5(BLOCK_SIZE * block_index);
     const uint64_t MAX_INT_SIZE =
@@ -680,8 +680,10 @@ FloatToString<long double>::get_positive_block(int block_index) {
       val = internal::get_table_positive<4096>(pos_exp, block_index);
     } else if (MAX_INT_SIZE < 8192) {
       val = internal::get_table_positive<8192>(pos_exp, block_index);
-    } else {
+    } else if (MAX_INT_SIZE < 16384) {
       val = internal::get_table_positive<16384>(pos_exp, block_index);
+    } else {
+      val = internal::get_table_positive<16384 + 128>(pos_exp, block_index);
     }
 #endif
     const uint32_t shift_amount = SHIFT_CONST + pos_exp - exponent;
diff --git a/libc/src/__support/ryu_long_double_constants.h b/libc/src/__support/ryu_long_double_constants.h
index 8415423e9eddd6a..86640bcc8257007 100644
--- a/libc/src/__support/ryu_long_double_constants.h
+++ b/libc/src/__support/ryu_long_double_constants.h
@@ -35403,6 +35403,558 @@ constexpr uint64_t POW10_SPLIT[][5] = {
     {8185529170695989288u, 251935u, 0u, 0u, 0u},
     {4647388653739187u, 0u, 0u, 0u, 0u},
     {4647389u, 0u, 0u, 0u, 0u},
+    {1u, 0u, 0u, 0u, 1666406u},
+    {9167511163728910775u, 3312060066959522888u, 1295184565790726114u, 14514316166628409175u, 467615u},
+    {18446650963712978694u, 11459123952047835623u, 11203169122561999106u, 12474589757305876786u, 2263011u},
+    {1727375103581005166u, 6279996614298463960u, 7709863957368393064u, 16687049419988932499u, 1149488u},
+    {5260295235157421525u, 18426378431810315116u, 1049713953025921878u, 12343052908124526721u, 3504815u},
+    {3705581585852188490u, 3335423218796647794u, 5009356519446298754u, 17358475006776455890u, 3071434u},
+    {14296650234967904607u, 13533728612605645886u, 13550853390885990417u, 17206365355322613797u, 2749915u},
+    {18409334644427041092u, 1794178469698007910u, 17611594257294438978u, 1708051658308162884u, 3019512u},
+    {4754135742177139480u, 6231107125027989758u, 17105560207999383557u, 17277465141864323237u, 2595660u},
+    {18358242171314739190u, 5662514190210093154u, 11287743438560124974u, 5452201045844425276u, 2058811u},
+    {17761256625928321051u, 17212837789613283598u, 9527998849657635872u, 11639251005171736777u, 1601676u},
+    {5821492835406745926u, 2889087753330502654u, 17816699099621606956u, 1830985569848452224u, 878190u},
+    {12963632653576508465u, 16198926776608324708u, 18116681369755801167u, 3835252232019257168u, 913381u},
+    {9574571567457348478u, 3277569375660351599u, 18148476178141229221u, 8447587411821609647u, 210151u},
+    {13880179963005905487u, 5477919832920232767u, 14817599233219952607u, 8578730300674845931u, 1381841u},
+    {4139500131013550121u, 3663479087884115807u, 3494753886518274538u, 15734045976124579229u, 3587513u},
+    {3951002731038455889u, 6648345471811276523u, 14225211067814220084u, 8568974046381648398u, 2873430u},
+    {1688486474885901699u, 3743680607006362896u, 5194811187761484099u, 15977733718774767139u, 2499029u},
+    {11994796996225841889u, 17880557183921429048u, 15497995200573975779u, 12079717168707477337u, 3113160u},
+    {14145368163067234658u, 9261433793631684194u, 7680189330107885166u, 13243967386801039084u, 526971u},
+    {15445030309197543459u, 14728335854035343397u, 17075675439650901028u, 7287537910245955718u, 2849008u},
+    {1142171144987344907u, 7490797843865302792u, 8787155366672039387u, 2718685908130822644u, 484791u},
+    {5408153414594581696u, 2427550704946339717u, 6793841772739607308u, 14348404031772582899u, 624735u},
+    {12520228542564573657u, 14812481887778795293u, 9249283428732450365u, 8081974873255221800u, 806845u},
+    {790425527012045027u, 17212573622941371702u, 7910578513568475163u, 12408789845827732048u, 1161789u},
+    {18054897266975082573u, 8506153697810646314u, 1662661719671814836u, 4056656502883405208u, 3185768u},
+    {9270379265643408620u, 1195149283137702515u, 13207984899139388752u, 9930657434226997265u, 2202773u},
+    {16614137443348276141u, 14781126627478736439u, 6803683723866315318u, 14668325589413505852u, 859112u},
+    {7447673545517676182u, 1083916344985410534u, 13308050152951963757u, 4195188288062798637u, 2406710u},
+    {640276865045725606u, 9429623404773127942u, 600293903614474160u, 1125259878193744843u, 2131145u},
+    {16798302994488502999u, 1110264672744614054u, 10559380483812768002u, 11568527733592695380u, 931863u},
+    {2248359540383920274u, 5894188707032120406u, 9043257816917527345u, 6934718877482368793u, 164401u},
+    {2862341250731230715u, 4246895923326563304u, 8809995434622158204u, 1876530115093307137u, 2901191u},
+    {8405103544676021516u, 11567689276855949105u, 15784543052813908722u, 4809318736265723378u, 999334u},
+    {15579631011428052726u, 16785760031102532158u, 5909361390067953776u, 9241806500206251005u, 2150196u},
+    {370319694131435420u, 3083801843464632004u, 4662741834226062242u, 6164559617785995043u, 3620301u},
+    {1033559633838940172u, 15810150688083630215u, 3891367580811375289u, 13037149699008382861u, 120482u},
+    {11308568505275257325u, 613731621304509155u, 3803382403086045620u, 18304851399290334117u, 421341u},
+    {8677144826867200054u, 17827139855619338965u, 8813322557930393339u, 8078222920147541087u, 3545930u},
+    {7375575360705793469u, 12651035560908367098u, 9394262074024389219u, 18440097350963135510u, 1843942u},
+    {16669835522062886782u, 17348663479059537611u, 18244674427130191192u, 1619281813435276080u, 747200u},
+    {17464778948620520076u, 12917510971872457916u, 7429449805763610688u, 15722338909059447638u, 491710u},
+    {7702674345127274593u, 11601590047702440914u, 12627737072937908926u, 15717625964519112680u, 1868229u},
+    {9788000944531531893u, 15462636986002358161u, 7842093577620732495u, 11851908180171889790u, 2452442u},
+    {12109297881698999285u, 17131651483582465886u, 6569550798974851021u, 1774621838691795044u, 753597u},
+    {13133653791234500424u, 4324827303956416464u, 12192519312904677579u, 6931430440409418991u, 1146093u},
+    {5045375477987867200u, 6678799442071584796u, 10139350272668939471u, 17603194636441516825u, 1872575u},
+    {12798246487239652604u, 16714555538748891233u, 1085554466557611808u, 14950464895238104052u, 720150u},
+    {2664578200625687471u, 16793883355150161601u, 15391344404818619526u, 17235049412759923532u, 1840075u},
+    {18374901552925357261u, 12553865988854528153u, 17519513554508473654u, 12139619208208373763u, 3447533u},
+    {17671381561613929399u, 2262727631311519342u, 15121480342923670975u, 3017957126631332695u, 837291u},
+    {8460112156141233331u, 7983046005719909202u, 15005410844269686945u, 7365319887678826942u, 2794384u},
+    {2379370595853479811u, 13860668065444366236u, 6673137095531534673u, 9779322488977260039u, 1224093u},
+    {1415933634462666168u, 13318864766636345134u, 2647925470075013075u, 13065005060937698251u, 2142779u},
+    {7782049967851326679u, 14165493027220141411u, 4424988645680867478u, 12505491077446057268u, 3817586u},
+    {14298756137794571746u, 5871188803067413627u, 14048602070109968112u, 7708527012447228845u, 3011900u},
+    {4050782139123098694u, 16341196407815195137u, 17698899155718192812u, 8414240664583773386u, 2285155u},
+    {5890934981019196614u, 7047030131107131070u, 126062594834924354u, 13661038951036378319u, 3048693u},
+    {6564357374484156534u, 13967226511753616903u, 1498174585292480796u, 17854464200559548937u, 2283376u},
+    {13259932982941242686u, 6889276820041754061u, 12733502488724171474u, 8256686590874299525u, 3495034u},
+    {9313261355332184729u, 13168455044449058327u, 16546258483028024409u, 11593687052052069739u, 2491259u},
+    {13207118334093637735u, 3391170805209232789u, 16627789664552178912u, 4729699241253328475u, 71313u},
+    {7085701162826158826u, 15325670087512086235u, 9365786138160176070u, 5693865426386134634u, 267597u},
+    {14198705572034952613u, 14562521029100056325u, 4979170736928754471u, 2887240060584875321u, 826036u},
+    {12613313478282210826u, 7767769415626507749u, 14085002163747829797u, 18173751375132750675u, 554120u},
+    {3568965762911820816u, 943907302893284828u, 16126302233087205720u, 14709970911737174255u, 3795586u},
+    {16009388400392220742u, 11870203406362803303u, 16918269583830235138u, 8933100284926861981u, 1843037u},
+    {12360163387826619807u, 7286063737319541239u, 14915295217745959906u, 610458793093901203u, 243975u},
+    {4678693883975533460u, 8650260372010974321u, 17513611905647666644u, 2670631524399175712u, 3709456u},
+    {16140389596301619804u, 14018514106585446252u, 16573599126622797627u, 2374270397369011814u, 3456412u},
+    {17824937920199406379u, 15949883623231815687u, 1644962704585409640u, 3234293687620398160u, 1623083u},
+    {7450515525907448647u, 9803984207968383106u, 14498223819039278025u, 14729689783682981351u, 1011383u},
+    {16773139797707794461u, 6386168526813270247u, 2306327785699430784u, 18393343217762814051u, 1794415u},
+    {2118816782039041761u, 8371232817323324030u, 15234034981350631406u, 10049106703932351846u, 3740848u},
+    {12707956644941545515u, 11976903329115696638u, 12469301980390122684u, 13327603778702495156u, 1254629u},
+    {9039950576108104416u, 17209421422031733581u, 13593878325969039004u, 12921453166187158463u, 1521546u},
+    {11248333658881489667u, 16727437224260833846u, 11257619938869563541u, 1036873899110820743u, 2318833u},
+    {17879843874279495826u, 4181601617475580607u, 15000982146088610352u, 3861827403947726647u, 824110u},
+    {13578648902386511104u, 13590197159220965458u, 9776560520962997987u, 15795815244426630166u, 3725688u},
+    {7017447564488921474u, 16338797213553232755u, 6783642089315312534u, 16641973457553731316u, 3755983u},
+    {17446940937448854982u, 10737119951556083111u, 6660194076904400201u, 8860312146415385472u, 3473657u},
+    {16519166898418538210u, 17091600978035266948u, 6514747771652737435u, 5252224441269973238u, 1919721u},
+    {4417044329409092931u, 11380051351719377962u, 7522441564903196811u, 4214753061431970503u, 1841622u},
+    {5491857990967617848u, 9024171026985363161u, 5499558347578912177u, 3997139606015302673u, 348159u},
+    {6994480456139902559u, 12313040250767907698u, 14690574092329243413u, 13120904518869982601u, 2135085u},
+    {1749605835261128324u, 9540847690459802115u, 16270580970601591029u, 471730943919088292u, 3546644u},
+    {3749285147232329917u, 18225836298634055362u, 16614336693631217981u, 10369659982083983330u, 906236u},
+    {2743226272316612352u, 494720978589131697u, 5575725903735101804u, 5348979072738709495u, 902588u},
+    {3368311441953910552u, 2002280659113327640u, 9457492639531033449u, 737225755568259789u, 3541191u},
+    {12026781733668095552u, 11397995406927758609u, 6730745515409304376u, 3524087958650890284u, 2961094u},
+    {951703565280633457u, 10944382877554487105u, 12612374439307178584u, 2432523149971906757u, 3258871u},
+    {3678049391738471619u, 6893069984109299175u, 2883083942276601277u, 10148178727048668110u, 1798332u},
+    {8881561970967635567u, 11415923064514023299u, 7846739896879877179u, 9977121357545796140u, 3435576u},
+    {15140395773232853756u, 15949019427562859500u, 894426476091955682u, 17429255364335532700u, 3067248u},
+    {17343315567527744615u, 11986546187690574179u, 2040310762243297928u, 12450486930819457831u, 1902443u},
+    {17947624836606589070u, 18081310986542907202u, 5985151613684732328u, 14302497511096038479u, 3638956u},
+    {1795629144512604728u, 6353876384338179960u, 3807248879631167322u, 3670006606226384126u, 2669315u},
+    {8677459242106997072u, 8003813114481567210u, 18414409997315103887u, 15253392516329915113u, 2568061u},
+    {4549848593366540956u, 14148344854827193065u, 2996704749739267152u, 4875231179827238853u, 215866u},
+    {1258411078263355085u, 8071640287602051658u, 16535261262344243984u, 9515584442737934117u, 3168614u},
+    {4530894777772685112u, 2537700881417552542u, 17010007271659599876u, 15622890933234391735u, 283124u},
+    {14658436266069083432u, 1958346567304787567u, 6015854389490711016u, 16362296578201657348u, 593706u},
+    {2401778765968598525u, 7251070512154274413u, 4674151200882376820u, 155067147075178253u, 275922u},
+    {14207166018946484512u, 4204863376551275335u, 18007125055201591893u, 13551917541803825201u, 10119u},
+    {4936749078018473750u, 1556720238523122447u, 13957313961871519189u, 11817632098375380912u, 1612084u},
+    {1893466179509874520u, 18326974623317736743u, 3370379831240751071u, 7811957868887171175u, 1770409u},
+    {14926971438317492437u, 11182884361940575192u, 8376705575892579410u, 8031051227750750818u, 844059u},
+    {11508141943287493176u, 14962914172118026658u, 3287212790684287614u, 5491947295269685574u, 2068611u},
+    {12203518853080004698u, 3617089502073706471u, 10101639049898170375u, 18124615246716919620u, 483134u},
+    {10458294034992519390u, 16322774878186535045u, 4194128554942565631u, 15357179797454573205u, 606813u},
+    {7272475181693406686u, 15474321324440527285u, 11055742709930368211u, 16007979615888781503u, 948952u},
+    {11027272155779304107u, 8347542244846677935u, 5790210415825921656u, 2971866446245259818u, 1432432u},
+    {14971231974885946753u, 12913247256233115751u, 12648377723538553512u, 17968764624922914430u, 2925732u},
+    {9096847586665120971u, 11332908202750209530u, 14600548915543739907u, 15041949807290037706u, 2634952u},
+    {12751314985995067672u, 15668349264919302017u, 13645428086455660608u, 1561815775028945593u, 566942u},
+    {11563193684702396635u, 1552781408958269503u, 11828492545255311661u, 11467615687570994639u, 1110816u},
+    {1400683406726104317u, 12208172938207697009u, 15843415876437138014u, 17746659083262869692u, 2383573u},
+    {5406308906466419666u, 5127688944296462729u, 6214222207037262306u, 5088000761513618700u, 1080264u},
+    {2389978042491125839u, 14948981118903029532u, 10708802086349539540u, 7513917138572547880u, 2639383u},
+    {11832158916618231760u, 1813043199960936153u, 838234966387375669u, 17126337817216337708u, 312398u},
+    {5700684272435441046u, 7801926537902043096u, 13319893546977646890u, 5193909513812287925u, 1753546u},
+    {1570101120788386704u, 17423072751416593387u, 3879687148014542261u, 11489504671507928427u, 3568293u},
+    {14464154804035843377u, 14124577894714444578u, 10559652613460736609u, 3452530319023126940u, 481637u},
+    {2189077236620091698u, 3171089786171651284u, 6176012944244790385u, 17590937583182375950u, 2553494u},
+    {18288199254312507011u, 11629802359215142600u, 18013656107996006429u, 17124753454891611312u, 1462544u},
+    {4889116951016786383u, 5998709352979101940u, 6461570099486319930u, 4926895586568392565u, 3554236u},
+    {2199975600337289595u, 5847296645763337637u, 8593193283912379856u, 16278522745330246328u, 639489u},
+    {1528656688250066267u, 7353460423402946370u, 14455063314795463713u, 3470561020020016120u, 3101567u},
+    {3309997611820274407u, 7165917856594695028u, 16546699786208060589u, 3804208706119276804u, 1191102u},
+    {7247262146298419974u, 8102554607163688212u, 8636721983754521603u, 886663086019027532u, 36260u},
+    {6985121815884159426u, 3339005953553563912u, 6652332961876980623u, 18159182577384615666u, 836459u},
+    {3775045753865620181u, 11298099168662134230u, 4054288322569509565u, 2825676130739523099u, 2424698u},
+    {15009540523965777788u, 5814927245346129690u, 2282509117946592630u, 17410607949428344109u, 2734920u},
+    {5714078105996605758u, 10456837455245123602u, 6160748199906420014u, 13741393253918985516u, 460852u},
+    {10698833280884522061u, 12003930615174384973u, 3695295091961330649u, 801134767058457744u, 2831945u},
+    {17314883983826999139u, 5091428160819785466u, 3867006901853908082u, 2213967986584794243u, 328921u},
+    {3874904667587726835u, 16276922674913825336u, 219158004127495052u, 14417586331307023806u, 2044083u},
+    {6462729573893065578u, 14810529203087500665u, 12966436553073377484u, 8612560380897431156u, 2220044u},
+    {10992882775436657320u, 5451928658754496942u, 13009054558269493127u, 5517329738995457964u, 3652553u},
+    {11276480754406500102u, 6345866629411296281u, 16217152760168926909u, 499723280151557398u, 1244174u},
+    {2050366996710390896u, 10268184946870732763u, 15197213283395228081u, 7228710363653680387u, 2510223u},
+    {6532193867484341533u, 7457984715706605113u, 8113814323310135212u, 15610745760670083951u, 934917u},
+    {16469990856175821417u, 10102528740535617311u, 9929141208155016529u, 4196586644439726361u, 3420949u},
+    {1415383548198264335u, 7610345376856824064u, 6104580011670919070u, 783681315268078621u, 896010u},
+    {15520506072182345160u, 10389412246267891887u, 11280751816407303678u, 14139816899375041266u, 480051u},
+    {2946249913431227664u, 8152608466925387942u, 6232300417495203228u, 11970416002375182619u, 2747316u},
+    {8125325724997209183u, 13396350733685016202u, 16281931502026287873u, 4590307471501483400u, 1267659u},
+    {16794466152993196567u, 1098757763778858028u, 18326293196870121102u, 1392478472456672831u, 1979936u},
+    {10368277562512346599u, 11193720681604274198u, 4520363714669583110u, 2198251195204640747u, 1673978u},
+    {7184967683087223506u, 8846546202861894722u, 18441010737938086033u, 5219026216680082754u, 2887387u},
+    {4835751726793421944u, 15196833033289030149u, 14608901680839924901u, 10861901999938972617u, 368653u},
+    {11890210545962808297u, 1289662455915373824u, 4996593304567863138u, 15643298364635269359u, 1316082u},
+    {3336554993796378285u, 2048191322387287071u, 13745736545843869859u, 16669581666238667394u, 1034001u},
+    {2727752792486717226u, 6581687957571454482u, 5887518377010538905u, 5567508709408992488u, 1316243u},
+    {12956757253818591693u, 13913997601160102501u, 13958488472393829671u, 9896170786523447622u, 3339119u},
+    {2767695869091116105u, 17414355213661054864u, 14866487952318986598u, 8132046415768760582u, 3062439u},
+    {7501146529116990161u, 5852935974416269097u, 5720556823733168865u, 7550481816550898765u, 3414693u},
+    {12756619068956600691u, 17942153926652316844u, 8987118190203140826u, 8205498101697626074u, 1816737u},
+    {17015936328127409039u, 10136771478937301710u, 6858924948191133665u, 16390586737303378443u, 2140237u},
+    {57068893624212306u, 4857076147447689907u, 17069721106543804309u, 3786475310558923318u, 23879u},
+    {13521054681577729046u, 3175496704063358748u, 17376127936839109697u, 4972414482205239004u, 1866179u},
+    {16667616643150715876u, 16238425037539080919u, 9784785207097591992u, 2412325534632767587u, 2535422u},
+    {4538245396420150238u, 4280696400899147636u, 7565185717904791440u, 46770283165178353u, 1370631u},
+    {14703604926721831111u, 11811609319397813971u, 11181826873128016813u, 9753058874443534239u, 2860867u},
+    {4988305735952570894u, 17806446327762073362u, 4547958265771955199u, 3655653393027376898u, 3539102u},
+    {456147882709526703u, 12714877307156564599u, 10526924677494593346u, 10297463265886173926u, 3147099u},
+    {9885549183067932984u, 4610282344598720356u, 1242128309267264411u, 11803441568307344090u, 2877109u},
+    {10305296386839065299u, 1637601500952734988u, 11947997347810065558u, 6970602332839689838u, 727840u},
+    {11108361518556214403u, 6048296423113065275u, 16378481821564012818u, 10822065390866401492u, 855476u},
+    {3205859866592529305u, 14406112921496106744u, 4513009014306327810u, 7437712943561843551u, 389110u},
+    {14355819235995762550u, 11385337577142860630u, 3526008637419628181u, 7933513164196307027u, 1555999u},
+    {2894580858817241479u, 16914896149213534922u, 14780479720941674...
[truncated]