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Date: Sun, 1 Aug 2021 17:59:52 +0200
From: "Stefan Kanthak" <stefan.kanthak@...go.de>
To: <musl@...ts.openwall.com>
Subject: [Patch] src/math/i386/remquo.s: remove conditional branch, shorter bit twiddling
<https://git.musl-libc.org/cgit/musl/plain/src/math/i386/remquo.s>
Halve the number of instructions (from 12 to 6) to fetch the
(3-bit partial) quotient from the FPU flags C0:C3:C1, and
perform its negation without conditional branch.
--- -/math/i386/remquo.s
+++ +/math/i386/remquo.s
@@ -2,49 +2,44 @@
.type remquof,@function
remquof:
mov 12(%esp),%ecx
+ mov 8(%esp),%eax
+ xor 4(%esp),%eax
flds 8(%esp)
flds 4(%esp)
- mov 11(%esp),%dh
- xor 7(%esp),%dh
- jmp 1f
+ jmp 0f
.global remquol
.type remquol,@function
remquol:
mov 28(%esp),%ecx
+ mov 24(%esp),%eax
+ xor 12(%esp),%eax
+ cwtl
fldt 16(%esp)
fldt 4(%esp)
- mov 25(%esp),%dh
- xor 13(%esp),%dh
- jmp 1f
+ jmp 0f
.global remquo
.type remquo,@function
remquo:
mov 20(%esp),%ecx
+ mov 16(%esp),%eax
+ xor 8(%esp),%eax
fldl 12(%esp)
fldl 4(%esp)
- mov 19(%esp),%dh
- xor 11(%esp),%dh
+0: cltd
1: fprem1
fnstsw %ax
sahf
jp 1b
fstp %st(1)
- mov %ah,%dl
- shr %dl
- and $1,%dl
- mov %ah,%al
- shr $5,%al
- and $2,%al
- or %al,%dl
- mov %ah,%al
- shl $2,%al
- and $4,%al
- or %al,%dl
- test %dh,%dh
- jns 1f
- neg %dl
-1: movsbl %dl,%edx
- mov %edx,(%ecx)
+ adc %al,%al
+ shl $2,%ah
+ adc %al,%al
+ shl $5,%ah
+ adc %al,%al
+ and $7,%eax
+ xor %edx,%eax
+ sub %edx,%eax
+ mov %eax,(%ecx)
ret
Download attachment "remquo.patch" of type "application/octet-stream" (1027 bytes)
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