Trace of CPU Instruction Reordering

问题:

I have studied a few things about instruction re-ordering by processors and Tomasulo's algorithm.

In an attempt to understand this topic bit more I want to know if there is ANY way to (get the trace) see the actual dynamic reordering done for a given program?

I want to give an input program and see the "out of order instruction execution trace" of my program.

I have access to an IBM-P7 machine and an Intel Core2Duo laptop. Also please tell me if there is an easy alternative.

回答1:

You have no access to actual reordering done inside the CPU (there is no publically known way to enable tracing). But there is some emulators of reordering and some of them can give you useful hints.

For modern Intel CPUs (core 2, nehalem, Sandy and Ivy) there is "Intel(R) Architecture Code Analyzer" (IACA) from Intel. It's homepage is http://software.intel.com/en-us/articles/intel-architecture-code-analyzer/

This tool allows you to look how some linear fragment of code will be splitted into micro-operations and how they will be planned into execution Ports. This tool has some limitations and it is only inexact model of CPU u-op reordering and execution.

There are also some "external" tools for emulating x86/x86_84 CPU internals, I can recommend the PTLsim (or derived MARSSx86):

PTLsim models a modern superscalar out of order x86-64 compatible processor core at a configurable level of detail ranging ... down to RTL level models of all key pipeline structures. In addition, all microcode, the complete cache hierarchy, memory subsystem and supporting hardware devices are modeled with true cycle accuracy.

But PTLsim models some "PTL" cpu, not real AMD or Intel CPU. The good news is that this PTL is Out-Of-Order, based on ideas from real cores:

The basic microarchitecture of this model is a combination of design features from the Intel Pentium 4, AMD K8 and Intel Core 2, but incorporates some ideas from IBM Power4/Power5 and Alpha EV8.

Also, in arbeit http://es.cs.uni-kl.de/publications/datarsg/Senf11.pdf is said that JavaHASE applet is capable of emulating different simple CPUs and even supports Tomasulo example.

回答2:

Unfortunately, unless you work for one of these companies, the answer is no. Intel/AMD processors don't even schedule the (macro) instructions you give them. They first convert those instructions into micro operations and then schedule those. What these micro instructions are and the entire process of instruction reordering is a closely guarded secret, so they don't exactly want you to know what is going on.