[PATCH] D151023: [llvm-exegesis] Add Target Memory Utility Functions

[Clement Courbet via Phabricator via llvm-commits]

courbet added inline comments.


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Comment at: llvm/tools/llvm-exegesis/lib/X86/Target.cpp:992
+                              .addReg(0));
+  // The below two instructions round to the nearest 4096 byte page.
+  GeneratedCode.push_back(
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should this be using `getpagesize()` ?


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Comment at: llvm/tools/llvm-exegesis/lib/X86/Target.cpp:978
 
+#ifdef __linux__
+std::vector<MCInst> ExegesisX86Target::generateLowerMunmap() const {
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aidengrossman wrote:
> courbet wrote:
> > All these are extremely complex. Can't we insert a call to a separate module instead ?
> > 
> > Given that we don't actually care about measuring the setup code, we don't need to inline  each function here.
> I agree that they're complex and the readability is not great.
> 
> Theoretically, we could do it in a separate module and call into that, but the requirements for setting up a deterministic execution environment with regard to memory require that all the code has to be in blocks of fixed size at known addresses. I don't see any feasible way to do that by just implementing these functions in C/C++ in the `llvm-exegesis` code base. It's theoretically possible to have an implementation that lives outside of the main `llvm-exegesis` code base and gets built/linked in to the JITed snippet and emitted to the same block of memory, but decided against that approach since there was still a lot of complexity with that method and it seemed to be a less prominent path.
> 
> Very open to suggestions to improve the readability/complexity though since I'm sure I haven't explored the solution space that thoroughly.
We discussed a couple solutions or this with Ondrej and Guillaume, for the record:

 - Writing the code in syscalls in asm or c++ in a separate module and call to that: We agree that this would be very hard to maintain two separate modules.
 - Writing this as textual asm here and using the snippet parsing facilities to transform the result to `MCInst`s: Still painful because there a a bunch of values that are not constant.

In the end we don;t really have a much better apporach than this oone, but we think that it could be made much more readable by refactoring, e.g. `generateSyscall(SYS_munmap, ...)` method, `roundTo(4096, X86::RSI);`


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  https://reviews.llvm.org/D151023/new/

https://reviews.llvm.org/D151023