[LLVMbugs] [Bug 21278] New: PowerPC long double literal conversion produces incorrectly rounded result

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=21278

            Bug ID: 21278
           Summary: PowerPC long double literal conversion produces
                    incorrectly rounded result
           Product: clang
           Version: trunk
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: hstong at ca.ibm.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Given
long double x = 123456.789L;


Clang produces the following:
x:
    .quad    4683220299150161609     # ppc_fp128
123456.788999999999999999999999999
    .quad    -4786497741563401076
    .size    x, 16


Which, in hexadecimal is:
40fe240c9fbe76c9
bd92f1a9fbe76c8c


However, the nearest result representable in the format is 1 ulp less in the
less significant double; calculations are below.

Sign and exponent for high double:
0x3ff + floor(ln(123456789 / 1000) / ln(2)) = 40f

Significand for high double:
123456789 / 1000 * 2^(-(0x40f - 0x3ff) + 52) = 8483885939586760 + 88/125
(round to 8483885939586761)
8483885939586761 in base 16 = 1e240c9fbe76c9

High double:
40fe240c9fbe76c9


Sign and exponent for low double:
0x800 + 0x3ff + floor(ln(-(123456789 / 1000 - 0x1e240c9fbe76c9 * 2^(-52 +
(0x40f - 0x3ff)))) / ln(2)) = bd9

Significand for low double:
-(123456789 / 1000 - 0x1e240c9fbe76c9 * 2^(-52 + (0x40f - 0x3ff))) * 2^(-(0xbd9
- 0x800 - 0x3ff) + 52) = 666532744850833408/125 = 5332261958806667 + 33/125
(round to 5332261958806667)
5332261958806667 in base 16 = 12f1a9fbe76c8b

Low double:
bd92f1a9fbe76c8b


## COMMAND TO REPRODUCE:
echo "long double x = 123456.789L;" |
clang -target powerpc64-unknown-linux-gnu -S -o - -x c -


## ACTUAL OUTPUT:
    .text
    .file    "-"
    .type    x, at object               # @x
    .data
    .globl    x
    .align    4
x:
    .quad    4683220299150161609     # ppc_fp128
123456.788999999999999999999999999
    .quad    -4786497741563401076
    .size    x, 16


    .ident    "clang version 3.6.0 (trunk 219523)"


## EXPECTED OUTPUT:
See issue description.


## COMPILER VERSION INFO: clang -target powerpc64-unknown-linux-gnu -v
clang version 3.6.0 (trunk 219523)
Target: powerpc64-unknown-linux-gnu
Thread model: posix

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