[LLVMdev] [RFC] CodeGen Context

[Bob Wilson]

On Oct 13, 2013, at 8:23 PM, Bill Wendling <isanbard at gmail.com> wrote:

> On Oct 13, 2013, at 2:35 PM, Bob Wilson <bob.wilson at apple.com> wrote:
> 
>> On Oct 12, 2013, at 1:55 AM, Bill Wendling <isanbard at gmail.com> wrote:
>> 
>>> CGContext ::
>>> 
>>> 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.
>> 
> I was using CGContext to keep this proposal separate from the current implementation. However, while there are some similarities with TargetMachine, there are two differences:
> 
> 1) A CGContext essentially contains a cache of "TargetMachine"'s --- one for each change in attributes that affect object generation.
> 
> 2) A TargetMachine needs to be derived for a specific machine (see LLVMTargetMachine). This is not how a context should work, in my opinion.
> 
> That said, I'm not that concerned what the final object is called. :-) If we keep the TargetMachine name, then it will definitely change in fundamental ways.

I guess the important point is that I think we should have a single object that handles all of this.  The current TargetMachine API does not do what we need.  Adding a new CGContext to wrap around a broken API is the wrong approach.  We should just fix the API.

> 
>>> 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.
>> 
> That's pretty much what I had in mind.
> 
>>> 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.
>> 
> I don't understand what you're saying here. The whole reason for this proposal is because the options are changing between functions. I didn't mention that it would return information only about the current function, though that will be the non-parallel case. If you just want an API that takes the function as an argument, that's fine with me. :)

Yes, I want APIs that take the function as an argument.  “Simple” options can be queried directly from the MachineFunctionInfo.  TargetMachine (or whatever we end up calling it) should have APIs to handle the more complicated target queries or things that are true for all functions.  E.G., I think it will be a while before we support multiple target triples in the same compilation, if ever, so it makes sense to keep the triple in TargetMachine for now.

> 
>>> 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 back-end:
>>> 
>>> 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.
> 
> 
> No, the file1.c should keep the `-mavx' flag. If you do a quick experiment, compiling file1.o with a different flag (try `-fstack-protector' and `-fstack-protector-all') won't affect the attributes it was originally compiled with.
> 
> $ cat f.c
> void bar(char *);
> 
> void foo() {
>  char b[37];
>  bar(b);
> }
> 
> $ clang -S -o - -emit-llvm f.c
> ; ModuleID = 'f.c'
> target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
> target triple = "x86_64-apple-macosx10.8.0"
> 
> define void @foo() nounwind ssp uwtable {
>  %b = alloca [37 x i8], align 16
>  %1 = getelementptr inbounds [37 x i8]* %b, i32 0, i32 0
>  call void @bar(i8* %1)
>  ret void
> }
> 
> declare void @bar(i8*)
> $ clang -S -o f.ll -emit-llvm f.c && clang -S -emit-llvm f.ll -o - -fstack-protector-all
> ; ModuleID = 'f.ll'
> target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
> target triple = "x86_64-apple-macosx10.8.0"
> 
> define void @foo() nounwind ssp uwtable {
>  %b = alloca [37 x i8], align 16
>  %1 = getelementptr inbounds [37 x i8]* %b, i32 0, i32 0
>  call void @bar(i8* %1)
>  ret void
> }
> 
> declare void @bar(i8*)

So what do you mean by "Options specified by function attributes may be overridden by options
specified in the OptionContext”?  That whole paragraph does not make sense to me.