id summary reporter owner description type status priority milestone component version resolution keywords cc 279 PL/I generates incorrect code for divide operation specifying truncate or floor of quotient Gary Dixon Eric Swenson "I am trying to implement an algorithm that wants the integer part of a division result, using float dec(59) data variables for dividend, divisor and quotient. The PL/I compiler generates code that ALWAYS rounds up the quotient, no matter what builtin functions surround the divide operation. {{{ bad_divide: proc(); dcl (dividend, divisor, product, quotient, quotient_t, quotient_f, quotient_d) float dec(59); dcl (divide, floor, trunc) builtin; dcl ioa_ entry() options(variable); divisor = 8.; dividend = 8.0779356694631608874161005084957309918536338955163955688476e+058; quotient = dividend / divisor; product = divisor * quotient; if product > dividend then call ioa_( ""quotient of divide was rounded up.""); dividend = 8.0779356694631608874161005084957309918536338955163955688476e+058; quotient_t = trunc( dividend / divisor ); product = divisor * quotient_t; if product > dividend then call ioa_( ""quotient using trunc was rounded up.""); dividend = 8.0779356694631608874161005084957309918536338955163955688476e+058; quotient_f = floor( dividend / divisor ); product = divisor * quotient_f; if product > dividend then call ioa_( ""quotient using floor was rounded up.""); dividend = 8.0779356694631608874161005084957309918536338955163955688476e+058; quotient_d = divide( dividend, divisor, 59 ); product = divisor * quotient_d; if product > dividend then call ioa_( ""quotient using divide was rounded up.""); end bad_divide; }}} Results from the bad_divide program prove that the quotient is being rounded up, because the divisor * quotient value is greater than the dividend. {{{ bad_divide quotient of divide was rounded up. quotient using trunc was rounded up. quotient using floor was rounded up. quotient using divide was rounded up. }}} In each case, the PL/I compiler generates code which does the divide operation, and is followed by an MVN to move the temporary result WITH ROUNDING to the quotient variable. It then calls whatever built-in function was specified to process this rounded-up result. Code for the simplest division (without a surrounding built-in function) is shown below. {{{ STATEMENT 1 ON LINE 11 quotient = dividend / divisor; 000104 aa 100 100 227 500 dv3d (pr),(pr),(pr) 000105 aa 6 00120 00 0075 desc9fl pr6|80,61 divisor 000106 aa 6 00100 00 0075 desc9fl pr6|64,61 dividend 000107 aa 6 00260 00 0077 desc9fl pr6|176,63 000110 aa 000 300 300 500 mvn (pr),(pr),round 000111 aa 6 00260 00 0077 desc9fl pr6|176,63 000112 aa 6 00160 00 0075 desc9fl pr6|112,61 quotient }}} Use of eis_tester shows that the dv3d instruction is generating the expected quotient with two fractional places added to the end to accommodate possible rounding. But the compiler ALWAYS adds a mvn instruction with ,round specified instead of conditionally doing the rounding unless suppressed by use of a trunc or floor built-in function. The following eis_tester script shows the dv3d instruction and its result. {{{ bad_divide_et_script 02/20/22 1431.5 pst Sun inst dv3d -nt ""divide using 3 arguments"" -mf1 ar -mf2 ar -mf3 ar; desc 1 -sd f -nn 61; /* float dec(59) divisor */ desc 2 -sd f -nn 61; /* float dec(59) dividend */ desc 3 -sd f -nn 63; /* float dec(61) quotient */ data 1 ""+"" (58) ""0"" ""8"" 000; /* divisor = 8.0 */ data 2 ""+80779356694631608874161005084957309918536338955163955688476"" 000; /* dividend = 8.0779356694631608874161005084957309918536338955163955688476e+058 */ data 3 ""+10097419586828951109270125635619663739817042369395494461060"" 000; /* quotient = 1.0097419586828951109270125635619663739817042369395494461060e+058 */ }}} Results of this script are shown below: {{{ eis_tester bad_divide_et_script -long ET TEST 1 (dv3d) Test Description: divide using 3 arguments Eis instruction: ( 354|4000 ) Ind Desc. - - - -- - - - 100100227500 100010000075 200020000075 300030000077 Pointer Registers: ( 354|20 ) pr0 - pr3 77777|1 361|1770 362|1760 363|1750 pr4 - pr7 77777|1 77777|1 77777|1 77777|1 Test Indicators: ( 354|111 ) 000000000200 This test will take 0 page faults. data field 1 ( 361|2000 ) 053060060060 060060060060 060060060060 060060060060 060060060060 060060060060 060060060060 060060060060 Previous line repeated 1 time. 060060060060 060060060060 060060060070 000 data field 2 ( 362|2000 ) 053070060067 067071063065 066066071064 066063061066 060070070067 064061066061 060060065060 070064071065 067063060071 071061070065 063066063063 070071065065 061066063071 065065066070 070064067066 000 data field 3 ( 363|2000 ) Result data field initialized to all zero bits. test data ( 354|23776 ) xxxxxxxxxxxx xxxxxxxxxxxx 053061060060 071067064061 071065070066 070062070071 065061061060 071062067060 061062065066 063065066061 071066066063 067063071070 061067060064 062063066071 063071065064 071064064066 061060066060 000xxxxxxxxx xxxxxxxxxxxx xxx Data resulting from test ( 1 - dv3d ) is incorrect. result data ( 363|1776 ) xxxxxxxxxxxx xxxxxxxxxxxx 053061060060 071067064061 071065070066 070062070071 065061061060 071062067060 061062065066 063065066061 071066066063 067063071070 061067060064 062063066071 063071065064 071064064066 061060065071 065060376xxx xxxxxxxxxxxx xxx *** TEST NOTES: divide using 3 arguments *** }}} Note that the result data represents the value: {{{ +1009741958682895110927012563561966373981704236939549446105950e-02 }}} If the trunc built-in was permitted to remove the fractional digits before rounding, this correct integer value would be obtained: {{{ +10097419586828951109270125635619663739817042369395494461059 }}} " defect new major Administration