64-bit Verilog Simulation on Linux and Windows

We recently released a 64-bit Linux version of the simulation engine. The main reason for this port was to allow the simulator to handle extremely large designs (32-bit applications are limited to 3GB on Linux and while this is enough for most designs, it’s a definite limitation for some of today’s large designs).

However, we were surprised and pleased to find that the 64bit simulator runtime executes about 30% faster than the 32bit version running on equivalent hardware. We also see a decent speedup during the simulation generation process (e.g. compiling the Verilog code into a simulation). 64bit mode does use a little more memory since pointers are 64bits rather than 32bits in size, and this had been one area of concern to us when starting the port, but in practice we found this had little impact on memory usage by the simulator.

We’ve not had a chance yet to fully explore all the reasons for the speedup, but initial research has turned up several reasons for this. First, in 64bit mode, about twice as many registers are available to the compiler, allowing commonly used varaiables to be kept inside the CPU longer. In addition, since 64bit Linux is less register constrained, it uses a different function calling convention (known as the __fastcall convention)where most function parameters can be passed in registers rather than on the stack, reducing the overhead of function calls. And, finally, 64bit integer operations (such as time value calculations in Verilog) can be done in a single operation, of course.

If you’re like me, you’d like to know more about exactly why we see such a dramatic speedup. Well, it’s on the list for further investigation, but that work is pending right now till we finish our 64bit Windows port. We’ll revisit this subject then, and in the meantime I hope to be able to report on what kind of improvement we see for 64bit Windows. We’re expecting less improvement here, since the 64bit Windows version of the fast-call covention only passes the first four parameters of a function via registers.

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