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Re: gcc -S vs clang -S

On Wed, May 13, 2015 at 08:41:39AM -0600, Martin Sebor wrote:
> On 05/12/2015 07:40 PM, Andrew Pinski wrote:
> >On Tue, May 12, 2015 at 6:36 PM, Fei Ding <> wrote:
> >>I think Thiago and Eric just want to know which code-gen is better and why...
> >
> >
> >You need to understand for a complex process (CISC ISAs) like x86,
> >there is no one right answer sometimes.  You need to look at each
> >micro-arch and understand the pipeline.  Sometimes different code
> >stream will performance the same but it also depends on the code size
> >too.
> A good place to start is the Intel 64 and IA-32 Architectures
> Optimization Reference Manual. It lists the throughput and
> latencies of x86 instructions and gives guidance for which
> ones might be more efficient on which processors. For example,
> in the section titled Using LEA it discusses why the three
> operand form of the instruction is slower on the Sandy Bridge
> microarchitecture than on others:

But leal (%rdi,%rsi), %eax is not the slower case it talks about.
Furthermore, supposedly the generic tuning is used, in which case it really
doesn't matter that much if it is slower or faster on a particular CPU,
but if it is in general slower or faster on the whole basket of CPUs that
the generic tuning is based on.


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