3 ms·

That's basically how the original qemu worked. It used GCC to generate target machine code "snippets" for each source instruction, and then simply stitched the

by rwmj 3y ago

That's basically how the original qemu worked. It used GCC to generate target machine code "snippets" for each source instruction, and then simply stitched them together. (Nowadays the translation is a bit more complex and works at the basic block level.)

dragontamer 3y ago
Wild tangent. I've got a gist of what a basic block is due to online discussions. I know it is roughly a list of instructions and that jumps are the boundaries. But I'm looking to understand it on a more fundamental level. I'm guessing the subject is SSA / single static assignment and compiler theory? Just wanna ask publicly if I'm on the right track, and if I'm following this discussion correctly.
- rwmj 3y ago
  It's definitely a fundamental concept of compiler theory. Wikipedia has a reasonable description: https://en.wikipedia.org/wiki/Basic_block https://en.wikipedia.org/wiki/Basic_block
- pm215 3y ago
  It's not specific to SSA, though SSA assumes it's working with basic blocks (as do a lot of compiler algorithms). And yeah, writeups of compiler theory (and compiler practice) are where to go for more detailed discussion. The core idea I think is that control flow complicates the kinds of analysis a lot of optimisation and other compiler passes need to do; so by first breaking the input into pieces which have very restricted possible flows of control ("only entered at the top, only exited at the bottom" is the classic definition) then: 1. You can get quite a long way with optimisations that only work within a basic block, and the restriction to within-a-BB simplifies things a lot because it can entirely ignore control flow 2. Analysis that only cares about control flow can work on a graph of BBs and ignore the individual-statement/instruction details, because the BB graph captures all the detail of what can branch where
- mhh__ 3y ago
  Read the book "Engineering a compiler" if you want to get a medium-hard but nicely written intro to basically all of the basics. Skip the bit about parsing, it doesn't matter for what you want to learn.

by rwmj 3y ago

dragontamer 3y ago

Wild tangent. I've got a gist of what a basic block is due to online discussions. I know it is roughly a list of instructions and that jumps are the boundaries. But I'm looking to understand it on a more fundamental level. I'm guessing the subject is SSA / single static assignment and compiler theory? Just wanna ask publicly if I'm on the right track, and if I'm following this discussion correctly.

rwmj 3y ago
It's definitely a fundamental concept of compiler theory. Wikipedia has a reasonable description: https://en.wikipedia.org/wiki/Basic_block https://en.wikipedia.org/wiki/Basic_block
pm215 3y ago
It's not specific to SSA, though SSA assumes it's working with basic blocks (as do a lot of compiler algorithms). And yeah, writeups of compiler theory (and compiler practice) are where to go for more detailed discussion. The core idea I think is that control flow complicates the kinds of analysis a lot of optimisation and other compiler passes need to do; so by first breaking the input into pieces which have very restricted possible flows of control ("only entered at the top, only exited at the bottom" is the classic definition) then: 1. You can get quite a long way with optimisations that only work within a basic block, and the restriction to within-a-BB simplifies things a lot because it can entirely ignore control flow 2. Analysis that only cares about control flow can work on a graph of BBs and ignore the individual-statement/instruction details, because the BB graph captures all the detail of what can branch where
mhh__ 3y ago
Read the book "Engineering a compiler" if you want to get a medium-hard but nicely written intro to basically all of the basics. Skip the bit about parsing, it doesn't matter for what you want to learn.