Date: Mon, 15 May 2000 02:39:59 -0700 (PDT) From: neis@cdc.informatik.tu-darmstadt.de To: freebsd-gnats-submit@FreeBSD.org Subject: i386/18560: libm's log1p not working as designed on Intel architectures. Message-ID: <20000515093959.8AE2137B52F@hub.freebsd.org>
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>Number: 18560 >Category: i386 >Synopsis: libm's log1p not working as designed on Intel architectures. >Confidential: no >Severity: non-critical >Priority: low >Responsible: freebsd-bugs >State: open >Quarter: >Keywords: >Date-Required: >Class: sw-bug >Submitter-Id: current-users >Arrival-Date: Mon May 15 02:40:03 PDT 2000 >Closed-Date: >Last-Modified: >Originator: Stefan Neis >Release: Actually none. >Organization: >Environment: >Description: While "porting" current libm to OS/2 (i.e. recompiling and running various tests), I noticed that assembler code for log1p(x) is basically as follows: Compute x+1, than call fyl2x. This plainly contradicts the man page: > log1p(x) returns a value equivalent to `log (1 + x)'. It > is computed in a way that is accurate even if the value of > x is near zero. IMHO, you really need to use fyl2xp1, if x is sufficiently close to 0 for that instruction to work (unfortunately is working only in a rather small interval). On a related issue, the various wrapper functions around assembler code cause an additional function call which really causes a performance loss. I have been able to speed up e.g. "acos" by more than then percent by replacing the assembler routine __ieee754_acos with inline assembler code. >How-To-Repeat: >Fix: >Release-Note: >Audit-Trail: >Unformatted: To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-bugs" in the body of the message
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