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From a732e80d33b4fd6f510f7cec4f5573ef5d89bc4e Mon Sep 17 00:00:00 2001
From: Szabolcs Nagy <nsz@port70.net>
Date: Wed, 5 Nov 2014 21:40:29 +0100
Subject: [PATCH] math: fix x86_64 and x32 asm not to use sahf instruction
Some early x86_64 cpus (released before 2006) did not support sahf/lahf
instructions so they should be avoided (intel manual says they are only
supported if CPUID.80000001H:ECX.LAHF-SAHF[bit 0] = 1).
The workaround simplifies exp2l and expm1l because fucomip can be
used instead of the fucomp;fnstsw;sahf sequence copied from i386.
In fmodl and remainderl sahf is replaced by a simple bit test.
---
src/math/x32/exp2l.s | 13 +++----------
src/math/x32/fmodl.s | 4 ++--
src/math/x32/remainderl.s | 4 ++--
src/math/x86_64/exp2l.s | 13 +++----------
src/math/x86_64/fmodl.s | 4 ++--
src/math/x86_64/remainderl.s | 4 ++--
6 files changed, 14 insertions(+), 28 deletions(-)
diff --git a/src/math/x32/exp2l.s b/src/math/x32/exp2l.s
index d9f4d6e..dfb2bc7 100644
--- a/src/math/x32/exp2l.s
+++ b/src/math/x32/exp2l.s
@@ -6,9 +6,7 @@ expm1l:
fmulp
movl $0xc2820000,-4(%esp)
flds -4(%esp)
- fucomp %st(1)
- fnstsw %ax
- sahf
+ fucomip %st(1)
fld1
jb 1f
# x*log2e <= -65, return -1 without underflow
@@ -17,11 +15,8 @@ expm1l:
ret
1: fld %st(1)
fabs
- fucom %st(1)
- fnstsw %ax
+ fucomip %st(1)
fstp %st(0)
- fstp %st(0)
- sahf
ja 1f
f2xm1
ret
@@ -53,9 +48,7 @@ exp2l:
fld %st(1)
fsub %st(1)
faddp
- fucomp %st(1)
- fnstsw
- sahf
+ fucomip %st(1)
je 2f # x - 0x1p63 + 0x1p63 == x
movl $1,(%esp)
flds (%esp) # 0x1p-149
diff --git a/src/math/x32/fmodl.s b/src/math/x32/fmodl.s
index 9e4378a..b951320 100644
--- a/src/math/x32/fmodl.s
+++ b/src/math/x32/fmodl.s
@@ -5,7 +5,7 @@ fmodl:
fldt 8(%esp)
1: fprem
fstsw %ax
- sahf
- jp 1b
+ testb $4,%ah
+ jnz 1b
fstp %st(1)
ret
diff --git a/src/math/x32/remainderl.s b/src/math/x32/remainderl.s
index c97f68a..79bf4fe 100644
--- a/src/math/x32/remainderl.s
+++ b/src/math/x32/remainderl.s
@@ -5,7 +5,7 @@ remainderl:
fldt 8(%esp)
1: fprem1
fstsw %ax
- sahf
- jp 1b
+ testb $4,%ah
+ jnz 1b
fstp %st(1)
ret
diff --git a/src/math/x86_64/exp2l.s b/src/math/x86_64/exp2l.s
index 0d6cd56..0e9bdf9 100644
--- a/src/math/x86_64/exp2l.s
+++ b/src/math/x86_64/exp2l.s
@@ -6,9 +6,7 @@ expm1l:
fmulp
movl $0xc2820000,-4(%rsp)
flds -4(%rsp)
- fucomp %st(1)
- fnstsw %ax
- sahf
+ fucomip %st(1)
fld1
jb 1f
# x*log2e <= -65, return -1 without underflow
@@ -17,11 +15,8 @@ expm1l:
ret
1: fld %st(1)
fabs
- fucom %st(1)
- fnstsw %ax
+ fucomip %st(1)
fstp %st(0)
- fstp %st(0)
- sahf
ja 1f
f2xm1
ret
@@ -53,9 +48,7 @@ exp2l:
fld %st(1)
fsub %st(1)
faddp
- fucomp %st(1)
- fnstsw
- sahf
+ fucomip %st(1)
je 2f # x - 0x1p63 + 0x1p63 == x
movl $1,(%rsp)
flds (%rsp) # 0x1p-149
diff --git a/src/math/x86_64/fmodl.s b/src/math/x86_64/fmodl.s
index ca81e60..cd8d2b7 100644
--- a/src/math/x86_64/fmodl.s
+++ b/src/math/x86_64/fmodl.s
@@ -5,7 +5,7 @@ fmodl:
fldt 8(%rsp)
1: fprem
fstsw %ax
- sahf
- jp 1b
+ testb $4,%ah
+ jnz 1b
fstp %st(1)
ret
diff --git a/src/math/x86_64/remainderl.s b/src/math/x86_64/remainderl.s
index 75c1237..2c337cf 100644
--- a/src/math/x86_64/remainderl.s
+++ b/src/math/x86_64/remainderl.s
@@ -5,7 +5,7 @@ remainderl:
fldt 8(%rsp)
1: fprem1
fstsw %ax
- sahf
- jp 1b
+ testb $4,%ah
+ jnz 1b
fstp %st(1)
ret
--
2.2.0
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