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Fix i386 fdim double rounding (bug 20255).

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fdim suffers from double rounding on i386 because subtracting two
double values can produce an inexact long double value exactly half
way between two double values.  This patch fixes this by creating an
i386-specific version of fdim - C, based on the generic version,
unlike the previous .S version - which sets the x87 precision control
to double precision for the subtraction and then restores it
afterwards.  As noted in the comment added, there are no issues of
double rounding for subnormals (a case that setting precision control
does not address) because subtraction cannot produce an inexact result
in the subnormal range.

Tested for x86_64 and x86.

	[BZ #20255]
	* sysdeps/i386/fpu/s_fdim.c: New file.  Based on math/s_fdim.c.
	* math/libm-test.inc (fdim_test_data): Add another test.
This commit is contained in:
Joseph Myers
2016-06-14 16:41:50 +00:00
parent f4015c8a86
commit 228a78c21b
3 changed files with 57 additions and 0 deletions
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math/libm-test.inc
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@@ -7203,6 +7203,9 @@ static const struct test_ff_f_data fdim_test_data[] =
TEST_ff_f (fdim, 0, 9, 0, NO_INEXACT_EXCEPTION|ERRNO_UNCHANGED),
TEST_ff_f (fdim, -9, 0, 0, NO_INEXACT_EXCEPTION|ERRNO_UNCHANGED),
TEST_ff_f (fdim, 0, -9, 9, NO_INEXACT_EXCEPTION|ERRNO_UNCHANGED),
#if TEST_COND_binary64
TEST_ff_f (fdim, 1.0, -0x1.0000000000001p-53, 1.0, INEXACT_EXCEPTION|ERRNO_UNCHANGED, 0x1.0000000000001p0, INEXACT_EXCEPTION|ERRNO_UNCHANGED, 1.0, INEXACT_EXCEPTION|ERRNO_UNCHANGED, 0x1.0000000000001p0, INEXACT_EXCEPTION|ERRNO_UNCHANGED),
#endif
TEST_ff_f (fdim, min_subnorm_value, min_subnorm_value, 0, NO_INEXACT_EXCEPTION|ERRNO_UNCHANGED),
TEST_ff_f (fdim, -min_subnorm_value, -min_subnorm_value, 0, NO_INEXACT_EXCEPTION|ERRNO_UNCHANGED),
TEST_ff_f (fdim, min_subnorm_value, -min_subnorm_value, 2*min_subnorm_value, NO_INEXACT_EXCEPTION|ERRNO_UNCHANGED),