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0 pointschrisseaton9y ago0 comments

Obviously no backend is in SSA format after register allocation, but before allocation what do other compilers use if not SSA? Is there some kind of low-level LLVM and GCC IR separate from the front-end and middle one? That isn't in SSA?

I know that the C2 and Graal compilers are SSA all the way until register allocation is done.

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UniQP9y ago

You can stay in SSA form even after register allocation. In libFirm the assigned registers are just attributes of the values in the SSA representation. There was some additional discussion regarding this in [1].

GCC uses a separate representation (RTL) for their backend that is not in SSA form [2]. LLVM stays in SSA form for some backend phases but lowers the SSA form before register allocation (as also mentioned in [1]).

I took a quick look at the C2 and the Graal compiler. C2 seems to use LLVM's code generator (and thus lowers SSA before register allocation) but Graal seems to support SSA-based register allocation, nice.

[1] https://news.ycombinator.com/item?id=11210948

[2] https://en.wikibooks.org/wiki/GNU_C_Compiler_Internals/GNU_C...

zigzigzag9y ago

I think you didn't understand Chris' post.

C2 is the HotSpot Server compiler. It definitely does not use LLVM anywhere.

UniQP9y ago

Since there is also a programming language called C2, I took a look at the wrong compiler.

Can you provide a link to the source code?

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