[Bug math/17907] New: fma modifies and does not reset SSE rounding mode on 32bit machines

[simonbyrne at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=17907

            Bug ID: 17907
           Summary: fma modifies and does not reset SSE rounding mode on
                    32bit machines
           Product: glibc
           Version: 2.19
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: simonbyrne at gmail dot com

Created attachment 8091
  --> https://sourceware.org/bugzilla/attachment.cgi?id=8091&action=edit
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

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