| Summary: | fma modifies and does not reset SSE rounding mode on 32bit machines | ||
|---|---|---|---|
| Product: | glibc | Reporter: | Simon Byrne <simonbyrne> |
| Component: | math | Assignee: | Not yet assigned to anyone <unassigned> |
| Status: | RESOLVED DUPLICATE | ||
| Severity: | normal | CC: | simonbyrne |
| Priority: | P2 | Flags: | fweimer:
security-
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| Version: | 2.19 | ||
| Target Milestone: | --- | ||
| Host: | Target: | ||
| Build: | Last reconfirmed: | ||
| Attachments: | fma rounding mode test function | ||
This looks like it would have been a consequence of bug 16064, fixed for 2.20. *** This bug has been marked as a duplicate of bug 16064 *** (More specifically: i386 uses sysdeps/ieee754/ldbl-96/s_fma.c, which saves state with feholdexcept, sets the rounding mode (twice) with fesetround, and then restores state, with exceptions merged, with feupdateenv. This was the sort of pattern that was mishandled before bug 16064 was fixed, because fesetround sets the SSE rounding mode but the fenv_t functions used not to handle it.) Great, thanks for letting me know. |
Created attachment 8091 [details] fma rounding mode test function The software fma function modifies the rounding mode for SSE floating point instructions (to round-toward-zero), but does not reset it correctly, on 32bit (686) processors. The x87 rounding mode is unaffected. Attached is a sample program: compile on a 32bit machine with gcc: $ gcc fmacheck.c -o fmacheck -lm -msse2 -mfpmath=sse or clang: $ clang fmacheck.c -o fmacheck -lm When run as $ ./fmacheck 0x1.0000000000001p+0 it should give 0x1.ffffffffffffep-1 if rounding mode is the default (round-to-nearest). If the rounding mode has been incorrectly modified to be round-toward-zero, then you will get 0x1.ffffffffffffdp-1