| Summary: | __float128 loses precision under memcheck | ||
|---|---|---|---|
| Product: | [Developer tools] valgrind | Reporter: | John Reiser <jreiser> |
| Component: | memcheck | Assignee: | Julian Seward <jseward> |
| Status: | REPORTED --- | ||
| Severity: | normal | ||
| Priority: | NOR | ||
| Version: | 3.8.0 | ||
| Target Milestone: | --- | ||
| Platform: | Compiled Sources | ||
| OS: | Linux | ||
| Latest Commit: | Version Fixed In: | ||
| Attachments: | executable test case __exp10(__float128) for x86_64, compressed | ||
As this is an 80-bit FP problem, we should probably close this as a dup of 197915. |
Created attachment 82498 [details] executable test case __exp10(__float128) for x86_64, compressed __float128 manipulations can lose around 70 bits of precision (i.e., maintain only about 41 bits of precision) when run under memcheck using valgrind-3.8.1 on x86_64. The test case program below prints 3fbc79ca10c9242235d511e976394d7a ## normal execution 3fbc79ca10c9240e12445f2000000000 ## valgrind-3.8.1 The program uses Intel math library libimf and also libirc, A complete executable for x86_64 is attached (29KB after compression). A short diagnosis suggests that the problem occurs when __exp10 separates __float128 into two 80-bit floating point numbers, then operates extensively in 80-bit floating-point arithmetic. ----- test case (from Peter van Hoof, [valgrind-users] 2013-09-24, "mysterious failures when using valgrind") #include <iostream> #include <iomanip> using namespace std; extern "C" { __float128 __exp10q(__float128); } struct f128 { union { __float128 f; long long x[2]; }; }; int main() { f128 x, y; x.x[0] = 0x0; x.x[1] = 0xc003400000000000ULL; // __float128(-20) y.f = __exp10q(x.f); cout << hex << setfill('0'); cout << setw(16) << y.x[1] << setw(16) << y.x[0] << endl; return 0; } ----- end test case