This is my proposal for how to solve the problem we have with function attributes that affect code generation changing between functions. (This is mostly a problem for LTO.)
Please take a look at this proposal, and let me know if you have any questions or comments.
Thanks, Bill. I do have some comments, but first I want to apologize for not sending them earlier. Bill had asked for feedback before sending this out to a wider audience but I didn’t get to it until now. Sorry Bill!
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A central repository for back-end objects. The back-end objects may change, so
they should not be "cached" by individual passes. This is analogous to the
current TargetMachine object. The term "CGContext" is used because it
separates the current implementation from the "ideal" implementation.
I’m pretty strongly opposed to introducing this new CGContext thing. We already have TargetMachine to collect all the target-specific information. With function attributes, the target info may depend on which function is being compiled, so we should just fix the TargetMachine APIs where necessary to let you specify the function. You describe the proposed CGContext as being analogous to TargetMachine, so let’s just keep TargetMachine and make it do what we need.
Important Options ::
Those options which affect back-end object construction.
So, the back-end has to be prepared for "important options" to change. The ideal
solution would be for the back-end to query the CGContext any time it needs
information on how to generate code. Unfortunately, this isn't currently
feasible, because of how back-end objects are constructed, though it is
something worth striving for. As such, there are four goals we want to achieve:
1. As many options as possible should be queried via the back-end directly
rather than relying upon objects holding onto these options,
I’m not quite sure what you mean by “queried via the back-end directly”. Here’s what I propose: When constructing a target-specific instance of MachineFunctionInfo, any “simple” function attributes that the target backend may need to query should be read from the IR (per goal 3 below) and recorded in the MachineFunctionInfo object. Target-independent function attributes can be handled in the MachineFunctionInfo base class. The information will then be retrieved as needed from the MachineFunctionInfo object.
2. Those which affect how objects are generated require those objects to be
regenerated when the important options change,
This whole notion of options “changing" is just wrong. It is inherently tied to a sequential compilation process where we handle one function at a time. See goal 4 below. We can’t have a single CGContext that just returns information about “the current function”. The right API should take the function as an argument.
3. There is no more dependence upon IR-level code. I.e., the back-end would
still function if the IR code were deleted, and
4. Not prevent the back-end from being parallelized.
Some things to note:
* Recreating the back-end for each changing set of important options is
expensive. A simple test showed that there is a measurable slowdown in the
worst-case scenario where the back-end is recreated for every function.
* Object creation in the back-end has a high order of coupling. I.e., one
object creates another object, which uses the original object, and may
create other objects dependent upon previous objects, etc.
* Most functions should have the same set of important options, thus reducing
the need to regenerate the back-end objects for each function.
* Some objects are created on demand, and may change during code generation.
This is a simple model of how command line options and function attributes will
be pass through the compiler from the front-end to the middle-end and finally
The front-end generates the functions with appropriate function attributes taken
from command line options. Because the front-end may be dealing with IR files
and the command line options that are currently used may be different from those
the function was generated with, the front-end will create an "OptionContext"
object. Options specified by function attributes may be overridden by options
specified in the OptionContext. These are used as IR options by the middle
end. A suitable API will be set up to make this transparent to the middle end
*waves hands wildly*.
I’m not sure I understand. If I do this:
$ clang -mavx -flto -c file1.c
$ clang -mno-avx file1.o file2.c
that the -mno-avx should win and that file1.c will be compiled without AVX support?
Maybe I missed some earlier discussion, but that seems really wrong to me. We need the front-end settings to be consistent with the code-gen options. Whatever options are specified when running the front-end should be preserved without overrides in the bit code.