Agner`s CPU blog

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Proposal now published
Author: Agner Date: 2016-03-24 14:50
Thanks for your comments. I know that I can't completely get rid of memory fragmentation and virtual address translation. I am just hoping to keep the number of memory sections and fragments so small that we can get rid of the fixed size memory pages and have a limited number of variable size memory blocks instead.

The instruction set makes sure that the data segment can be placed anywhere. It doesn't have to be adjacent to the code segment. Another thing is getting rid of the many DLLs or shared objects with each their code and data segment, but instead joining all DLL code segments into one.

Hubert Lamontagne wrote:

Any suggestions for an alternative to writing 64 consecutive save instructions or having a complex microcoded save-all instruction?
For general purpose registers, you're going to do 32 consecutive loads/stores so it's always going to take 'some' time.
I'd suggest mandating 16-byte stack alignment in the ABI, and having store-dual and load-dual instructions that can save/restore two registers in a single whole 128bit memory access.
I did consider 16-byte stack alignment for the sake of better alignment of vectors. But it will waste stack space on every call. Double push/pop instructions would have to be optional because I am trying to keep complexity down.

One solution is to have "save register and increment pointer" instructions in tiny form for both integer registers and full length vectors. Same for restore. Then you can save everything with 64 tiny instructions = 128 bytes of code.

About 32-bit index.

The compiler will probably have to add an instruction to sign-extend all 32bit indexes. It cannot replace 32bit variables with 64bit variables because that changes behavior - instead of 'int' being "32bits", it becomes "32bits except if the compiler decided to make it 64bits but it can still revert to 32bits at any time depending on if the value is in a register.
I just made an experiment to see what compilers actually do with a signed 32-bit index variable in 64-bit mode (x86-64). Both Ms and Gnu compilers simply replaced my 32-bit integer with a 64-bit integer. I don't think it is worth the effort to make a special addressing mode with sign-extended 32-bit index.
 
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