fix x87 stack imbalance in corner cases of i386 math asm

commit 31c5fb80b9 introduced underflow
code paths for the i386 math asm, along with checks on the fpu status
word to skip the underflow-generation instructions if the underflow
flag was already raised. unfortunately, at least one such path, in
log1p, returned with 2 items on the x87 stack rather than just 1 item
for the return value. this is a violation of the ABI's calling
convention, and could cause subsequent floating point code to produce
NANs due to x87 stack overflow. if floating point results are used in
flow control, this can lead to runaway wrong code execution.

rather than reviewing each "underflow already raised" code path for
correctness, remove them all. they're likely slower than just
performing the underflow code unconditionally, and significantly more
complex.

all of this code should be ripped out and replaced by C source files
with inline asm. doing so would preclude this kind of error by having
the compiler perform all x87 stack register allocation and stack
manipulation, and would produce comparable or better code. however
such a change is a much larger project.

src/math/i386/asin.s

@@ -7,13 +7,10 @@ asinf:
 	cmp $0x01000000,%eax
 	jae 1f
 		# subnormal x, return x with underflow
-	fnstsw %ax
-	and $16,%ax
-	jnz 2f
 	fld %st(0)
 	fmul %st(1)
 	fstps 4(%esp)
-2:	ret
+	ret
 
 .global asinl
 .type asinl,@function
@@ -30,11 +27,8 @@ asin:
 	cmp $0x00200000,%eax
 	jae 1f
 		# subnormal x, return x with underflow
-	fnstsw %ax
-	and $16,%ax
-	jnz 2f
 	fsts 4(%esp)
-2:	ret
+	ret
 1:	fld %st(0)
 	fld1
 	fsub %st(0),%st(1)

src/math/i386/atan.s

@@ -10,8 +10,5 @@ atan:
 	fpatan
 	ret
 		# subnormal x, return x with underflow
-1:	fnstsw %ax
-	and $16,%ax
-	jnz 2f
-	fsts 4(%esp)
-2:	ret
+1:	fsts 4(%esp)
+	ret

src/math/i386/atan2.s

@@ -10,8 +10,5 @@ atan2:
 	cmp $0x00200000,%eax
 	jae 1f
 		# subnormal x, return x with underflow
-	fnstsw %ax
-	and $16,%ax
-	jnz 1f
 	fsts 4(%esp)
-1:	ret
+	ret

src/math/i386/atan2f.s

@@ -10,10 +10,7 @@ atan2f:
 	cmp $0x01000000,%eax
 	jae 1f
 		# subnormal x, return x with underflow
-	fnstsw %ax
-	and $16,%ax
-	jnz 1f
 	fld %st(0)
 	fmul %st(1)
 	fstps 4(%esp)
-1:	ret
+	ret

src/math/i386/atanf.s

@@ -10,10 +10,7 @@ atanf:
 	fpatan
 	ret
 		# subnormal x, return x with underflow
-1:	fnstsw %ax
-	and $16,%ax
-	jnz 2f
-	fld %st(0)
+1:	fld %st(0)
 	fmul %st(1)
 	fstps 4(%esp)
-2:	ret
+	ret

src/math/i386/exp.s

@@ -7,13 +7,10 @@ expm1f:
 	cmp $0x01000000,%eax
 	jae 1f
 		# subnormal x, return x with underflow
-	fnstsw %ax
-	and $16,%ax
-	jnz 2f
 	fld %st(0)
 	fmul %st(1)
 	fstps 4(%esp)
-2:	ret
+	ret
 
 .global expm1l
 .type expm1l,@function
@@ -30,11 +27,8 @@ expm1:
 	cmp $0x00200000,%eax
 	jae 1f
 		# subnormal x, return x with underflow
-	fnstsw %ax
-	and $16,%ax
-	jnz 2f
 	fsts 4(%esp)
-2:	ret
+	ret
 1:	fldl2e
 	fmulp
 	mov $0xc2820000,%eax

src/math/i386/log1p.s

@@ -16,9 +16,6 @@ log1p:
 	fyl2x
 	ret
 		# subnormal x, return x with underflow
-2:	fnstsw %ax
-	and $16,%ax
-	jnz 1f
-	fsts 4(%esp)
+2:	fsts 4(%esp)
 	fstp %st(1)
-1:	ret
+	ret

src/math/i386/log1pf.s

@@ -16,10 +16,7 @@ log1pf:
 	fyl2x
 	ret
 		# subnormal x, return x with underflow
-2:	fnstsw %ax
-	and $16,%ax
-	jnz 1f
-	fxch
+2:	fxch
 	fmul %st(1)
 	fstps 4(%esp)
-1:	ret
+	ret