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| author | Joseph Myers <joseph@codesourcery.com> | 2012-04-09 09:42:05 +0000 |
|---|---|---|
| committer | Joseph Myers <joseph@codesourcery.com> | 2012-04-09 09:42:05 +0000 |
| commit | c483f6b4a4277bc209820efc1ae35d976af57b4e (patch) | |
| tree | 501d3025d498d3e27308f32db0838e19c13f826e /sysdeps/i386/fpu | |
| parent | d2de7579f257386ba5c28dfca94fa8aef143b4e0 (diff) | |
| download | glibc-c483f6b4a4277bc209820efc1ae35d976af57b4e.tar.gz glibc-c483f6b4a4277bc209820efc1ae35d976af57b4e.tar.xz glibc-c483f6b4a4277bc209820efc1ae35d976af57b4e.zip | |
Fix x86 pow inaccuracy for large integer exponents (bug 706).
Diffstat (limited to 'sysdeps/i386/fpu')
| -rw-r--r-- | sysdeps/i386/fpu/e_pow.S | 53 |
1 files changed, 49 insertions, 4 deletions
diff --git a/sysdeps/i386/fpu/e_pow.S b/sysdeps/i386/fpu/e_pow.S index b61a946082..73d2421162 100644 --- a/sysdeps/i386/fpu/e_pow.S +++ b/sysdeps/i386/fpu/e_pow.S @@ -32,6 +32,9 @@ limit: .double 0.29 ASM_TYPE_DIRECTIVE(p63,@object) p63: .byte 0, 0, 0, 0, 0, 0, 0xe0, 0x43 ASM_SIZE_DIRECTIVE(p63) + ASM_TYPE_DIRECTIVE(p10,@object) +p10: .byte 0, 0, 0, 0, 0, 0, 0x90, 0x40 + ASM_SIZE_DIRECTIVE(p10) .section .rodata.cst16,"aM",@progbits,16 @@ -116,7 +119,15 @@ ENTRY(__ieee754_pow) sahf jne 3f - /* OK, we have an integer value for y. */ + /* OK, we have an integer value for y. If large enough that + errors may propagate out of the 11 bits excess precision, use + the algorithm for real exponent instead. */ + fld %st // y : y : x + fabs // |y| : y : x + fcompl MO(p10) // y : x + fnstsw + sahf + jnc 2f popl %eax cfi_adjust_cfa_offset (-4) popl %edx @@ -157,7 +168,9 @@ ENTRY(__ieee754_pow) cfi_adjust_cfa_offset (8) .align ALIGNARG(4) -2: /* y is a large integer (so even). */ +2: // y is a large integer (absolute value at least 1L<<10), but + // may be odd unless at least 1L<<64. So it may be necessary + // to adjust the sign of a negative result afterwards. fxch // x : y fabs // |x| : y fxch // y : x @@ -187,9 +200,41 @@ ENTRY(__ieee754_pow) f2xm1 // 2^fract(y*log2(x))-1 : int(y*log2(x)) faddl MO(one) // 2^fract(y*log2(x)) : int(y*log2(x)) fscale // 2^fract(y*log2(x))*2^int(y*log2(x)) : int(y*log2(x)) - addl $8, %esp - cfi_adjust_cfa_offset (-8) fstp %st(1) // 2^fract(y*log2(x))*2^int(y*log2(x)) + testb $2, %dh + jz 292f + // x is negative. If y is an odd integer, negate the result. + fldl 20(%esp) // y : abs(result) + fld %st // y : y : abs(result) + fabs // |y| : y : abs(result) + fcompl MO(p63) // y : abs(result) + fnstsw + sahf + jnc 291f + + // We must find out whether y is an odd integer. + fld %st // y : y : abs(result) + fistpll (%esp) // y : abs(result) + fildll (%esp) // int(y) : y : abs(result) + fucompp // abs(result) + fnstsw + sahf + jne 292f + + // OK, the value is an integer, but is it odd? + popl %eax + cfi_adjust_cfa_offset (-4) + popl %edx + cfi_adjust_cfa_offset (-4) + andb $1, %al + jz 290f // jump if not odd + // It's an odd integer. + fchs +290: ret + cfi_adjust_cfa_offset (8) +291: fstp %st(0) // abs(result) +292: addl $8, %esp + cfi_adjust_cfa_offset (-8) ret |