From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeffrey A Law To: "Barry" Cc: egcs@cygnus.com Subject: Re: Penitum II & egcs 1.1.1 oddity Date: Fri, 22 Jan 1999 08:56:00 -0000 Message-id: <10010.917024018@hurl.cygnus.com> References: <000201be4624$e16f7e60$71aa989e@nibelung> X-SW-Source: 1999-01/msg00079.html In message < 000201be4624$e16f7e60$71aa989e@nibelung >you write: > > > Does it make a lot of difference compiling using -march=pentium as > > > opposed to -march=pentiumpro on a Pentium II? > > "a lot" is a relative term. > > > > -march=pentiumpro will enable ppro specific instructions, namely > > conditional > > moves, which for some code can be a nice improvement. For other > > code it won't > > help in any noticable way. Just depends on your application. > > > Thanks for the information. I'd be correct in assuming > that -march=pentiumpro would never cause a loss of performance > over -march=pentium? "never" is an extremely strong word. Given time I could probably come up with cases where cmoves could hurt. For example, by lengthening a basic block we could expose more loop invariants to the loop optimizer, which would pull them out the loop, increasing register pressure inside the loop, which could calls spills in the loop and possibly a net loss in performance. Having said that, I would expect any loss of performance due to using that option to be rare relative to the performance gains. jeff From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeffrey A Law To: "Barry" Cc: egcs@cygnus.com Subject: Re: Penitum II & egcs 1.1.1 oddity Date: Sun, 31 Jan 1999 23:58:00 -0000 Message-ID: <10010.917024018@hurl.cygnus.com> References: <000201be4624$e16f7e60$71aa989e@nibelung> X-SW-Source: 1999-01n/msg00843.html Message-ID: <19990131235800.t0Pm0vRaiKtDbdcqlql69FvxSbBoKTrdivgTidBPlrY@z> In message < 000201be4624$e16f7e60$71aa989e@nibelung >you write: > > > Does it make a lot of difference compiling using -march=pentium as > > > opposed to -march=pentiumpro on a Pentium II? > > "a lot" is a relative term. > > > > -march=pentiumpro will enable ppro specific instructions, namely > > conditional > > moves, which for some code can be a nice improvement. For other > > code it won't > > help in any noticable way. Just depends on your application. > > > Thanks for the information. I'd be correct in assuming > that -march=pentiumpro would never cause a loss of performance > over -march=pentium? "never" is an extremely strong word. Given time I could probably come up with cases where cmoves could hurt. For example, by lengthening a basic block we could expose more loop invariants to the loop optimizer, which would pull them out the loop, increasing register pressure inside the loop, which could calls spills in the loop and possibly a net loss in performance. Having said that, I would expect any loss of performance due to using that option to be rare relative to the performance gains. jeff