From baa43bca0a051e8deb0d6a9a8882ceeea5c27249 Mon Sep 17 00:00:00 2001 From: Rich Felker Date: Tue, 20 Mar 2012 00:51:32 -0400 Subject: optimize scalbn family the fscale instruction is slow everywhere, probably because it involves a costly and unnecessary integer truncation operation that ends up being a no-op in common usages. instead, construct a floating point scale value with integer arithmetic and simply multiply by it, when possible. for float and double, this is always possible by going to the next-larger type. we use some cheap but effective saturating arithmetic tricks to make sure even very large-magnitude exponents fit. for long double, if the scaling exponent is too large to fit in the exponent of a long double value, we simply fallback to the expensive fscale method. on atom cpu, these changes speed up scalbn by over 30%. (min rdtsc timing dropped from 110 cycles to 70 cycles.) --- src/math/i386/scalbn.s | 19 ++++++++++++++++--- src/math/i386/scalbnf.s | 18 +++++++++++++++--- src/math/i386/scalbnl.s | 16 +++++++++++++++- 3 files changed, 46 insertions(+), 7 deletions(-) (limited to 'src/math/i386') diff --git a/src/math/i386/scalbn.s b/src/math/i386/scalbn.s index e275d14f..8bf302f2 100644 --- a/src/math/i386/scalbn.s +++ b/src/math/i386/scalbn.s @@ -11,10 +11,23 @@ scalbln: .global scalbn .type scalbn,@function scalbn: - fildl 12(%esp) + mov 12(%esp),%eax + add \$0x3ffe,%eax + cmp \$0x7ffd,%eax + jb 1f + sub \$0x3ffe,%eax + sar \$31,%eax + xor \$0xfff,%eax + add \$0x3ffe,%eax +1: inc %eax fldl 4(%esp) - fscale - fstp %st(1) + mov %eax,12(%esp) + mov \$0x80000000,%eax + mov %eax,8(%esp) + xor %eax,%eax + mov %eax,4(%esp) + fldt 4(%esp) + fmulp fstpl 4(%esp) fldl 4(%esp) ret diff --git a/src/math/i386/scalbnf.s b/src/math/i386/scalbnf.s index 40232b6a..9cb9ef5f 100644 --- a/src/math/i386/scalbnf.s +++ b/src/math/i386/scalbnf.s @@ -11,10 +11,22 @@ scalblnf: .global scalbnf .type scalbnf,@function scalbnf: - fildl 8(%esp) + mov 8(%esp),%eax + add \$0x3fe,%eax + cmp \$0x7fd,%eax + jb 1f + sub \$0x3fe,%eax + sar \$31,%eax + xor \$0x1ff,%eax + add \$0x3fe,%eax +1: inc %eax + shl \$20,%eax flds 4(%esp) - fscale - fstp %st(1) + mov %eax,8(%esp) + xor %eax,%eax + mov %eax,4(%esp) + fldl 4(%esp) + fmulp fstps 4(%esp) flds 4(%esp) ret diff --git a/src/math/i386/scalbnl.s b/src/math/i386/scalbnl.s index 224b1bef..54414c2e 100644 --- a/src/math/i386/scalbnl.s +++ b/src/math/i386/scalbnl.s @@ -11,7 +11,21 @@ scalblnl: .global scalbnl .type scalbnl,@function scalbnl: - fildl 16(%esp) + mov 16(%esp),%eax + add \$0x3ffe,%eax + cmp \$0x7ffd,%eax + jae 1f + inc %eax + fldt 4(%esp) + mov %eax,12(%esp) + mov \$0x80000000,%eax + mov %eax,8(%esp) + xor %eax,%eax + mov %eax,4(%esp) + fldt 4(%esp) + fmulp + ret +1: fildl 16(%esp) fldt 4(%esp) fscale fstp %st(1) -- cgit v1.2.1