math: fix expm1l on x86_64 (avoid underflow for large negative x)

[oweals/musl.git] / src / math / x86_64 / exp2l.s

diff --git a/src/math/x86_64/exp2l.s b/src/math/x86_64/exp2l.s
index 510c256dec226bedbf22226aaafd4d1c44fc6024..0d6cd563baca544f6f49401a77939965ef2cee87 100644 (file)
--- a/src/math/x86_64/exp2l.s
+++ b/src/math/x86_64/exp2l.s
@@ -1 +1,90 @@
-# see expl.s
+.global expm1l
+.type expm1l,@function
+expm1l:
+       fldt 8(%rsp)
+       fldl2e
+       fmulp
+       movl $0xc2820000,-4(%rsp)
+       flds -4(%rsp)
+       fucomp %st(1)
+       fnstsw %ax
+       sahf
+       fld1
+       jb 1f
+               # x*log2e <= -65, return -1 without underflow
+       fstp %st(1)
+       fchs
+       ret
+1:     fld %st(1)
+       fabs
+       fucom %st(1)
+       fnstsw %ax
+       fstp %st(0)
+       fstp %st(0)
+       sahf
+       ja 1f
+       f2xm1
+       ret
+1:     push %rax
+       call 1f
+       pop %rax
+       fld1
+       fsubrp
+       ret
+
+.global exp2l
+.type exp2l,@function
+exp2l:
+       fldt 8(%rsp)
+1:     fld %st(0)
+       sub $16,%rsp
+       fstpt (%rsp)
+       mov 8(%rsp),%ax
+       and $0x7fff,%ax
+       cmp $0x3fff+13,%ax
+       jb 4f             # |x| < 8192
+       cmp $0x3fff+15,%ax
+       jae 3f            # |x| >= 32768
+       fsts (%rsp)
+       cmpl $0xc67ff800,(%rsp)
+       jb 2f             # x > -16382
+       movl $0x5f000000,(%rsp)
+       flds (%rsp)       # 0x1p63
+       fld %st(1)
+       fsub %st(1)
+       faddp
+       fucomp %st(1)
+       fnstsw
+       sahf
+       je 2f             # x - 0x1p63 + 0x1p63 == x
+       movl $1,(%rsp)
+       flds (%rsp)       # 0x1p-149
+       fdiv %st(1)
+       fstps (%rsp)      # raise underflow
+2:     fld1
+       fld %st(1)
+       frndint
+       fxch %st(2)
+       fsub %st(2)       # st(0)=x-rint(x), st(1)=1, st(2)=rint(x)
+       f2xm1
+       faddp             # 2^(x-rint(x))
+1:     fscale
+       fstp %st(1)
+       add $16,%rsp
+       ret
+3:     xor %eax,%eax
+4:     cmp $0x3fff-64,%ax
+       fld1
+       jb 1b             # |x| < 0x1p-64
+       fstpt (%rsp)
+       fistl 8(%rsp)
+       fildl 8(%rsp)
+       fsubrp %st(1)
+       addl $0x3fff,8(%rsp)
+       f2xm1
+       fld1
+       faddp             # 2^(x-rint(x))
+       fldt (%rsp)       # 2^rint(x)
+       fmulp
+       add $16,%rsp
+       ret