[PATCH] D77528: [MLIR] Add support to use aligned_alloc to lower AllocOp from std to llvm

[Alex Zinenko via Phabricator via llvm-commits]

ftynse added inline comments.


================
Comment at: mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp:1427
+      uint64_t constEltSizeBytes = 0;
+      auto isMallocAlignmentSufficient = [&]() {
+        if (auto vectorType = elementType.template dyn_cast<VectorType>())
----------------
nicolasvasilache wrote:
> bondhugula wrote:
> > ftynse wrote:
> > > bondhugula wrote:
> > > > ftynse wrote:
> > > > > I wouldn't add a lambda that is only called once immediately after its definition.
> > > > Hmm... this is just for better readability - it gives a name / auto documents a code block without the need to outline it into a function or add an explicit comment. I've seen this as a standard practice.
> > > This does not seem to be common practice in MLIR. FWIW, I find it less readable than just writing
> > > 
> > > ```
> > > int64_t constEltSizeBytes = 0;
> > > if (auto vectorType = elementType.template dyn_cast<VectorType>())
> > >   constEltSizeBytes =
> > >       vectorType.getNumElements() *
> > >       llvm::divideCeil(vectorType.getElementTypeBitWidth(), 8);
> > > else
> > >   constEltSizeBytes =
> > >       llvm::divideCeil(elementType.getIntOrFloatBitWidth(), 8);
> > > 
> > > // Use aligned_alloc if elt_size > malloc's alignment.
> > > bool isMallocAlignmentSufficient = constEltSizeBytes > kMallocAlignment;
> > > useAlignedAlloc = isMallocAlignmentSufficient;
> > > ```
> > > 
> > > Since you already have the block comment immediately above it anyway, and variables can have names just as well as lambdas. The lambda also mutates a global state that it captures by-reference, so the only effects of lambda are: (1) extra indentation; (2) extra LoC; and (3) extra concepts leading to cognitive overhead.
> > Hmm... the demarcation/isolation is important I feel. I'm fine with changing to the straightline style but out of curiosity and for future purposes as well, it'll be good to have a third person view here on coding style as far as such patterns go: @mehdi_amini - is there a guideline here?
> I am generally a fan of such style (esp. when mixed with functional combinators), so I'd vote for +1 it when it makes sense.
Generally, I would be strongly opposed to defining style guidelines based on a single use of a construct in a single diff, where only a small subset of contributors could participate (and before you can object that review history is public, are you reading all comments on all diffs?). I would be also opposed to having to define additional rules until we have to. I am not generally opposed to helper lambdas, I just don't see any benefit from this specific one, only drawbacks. And a lambda that the entire environment by reference is not exactly my definition of isolation.


================
Comment at: mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp:1589
+  // This is the alignment malloc typically provides.
+  constexpr static unsigned kMallocAlignment = 16;
 };
----------------
bondhugula wrote:
> ftynse wrote:
> > bondhugula wrote:
> > > ftynse wrote:
> > > > I am a bit concerned about hardcoding the "typical" value. Can we make it parametric instead?
> > > I had sort of a similar concern. But 16 bytes is pretty much what glibc malloc gives on nearly every system we have (on probably really old ones, it was perhaps 8 bytes). Did you want a pass flag and then letting 16 be the default - that would be too much plumbing (just like alignedAlloc). This is already a parameter of sorts. 
> > The world is not limited to glibc. MLIR should also work on other platforms, and you essentially shift the burden of the plumbing you didn't do to people debugging builds on those platforms. You can have one pass option that corresponds to malloc alignment and, if it is set to 0, treat it as "never use aligned_alloc".
> Sorry, I didn't quite understand. What should the pass options be and what should the behavior and the default behavior be?
Normally, you would have two pass options (and a configuration `struct` for the constructors like Nicolas proposed in another patch to decrease the amount of churn in pass constructor APIs): `-use-aligned-alloc` and `-assume-malloc-alignment`. If you don't want two separate options, you could get away with one `-use-aligned-alloc-and-assume-malloc-alignment` (did not think about a better name). If it is set to zero (default), the conversion doesn't use aligned_alloc at all. If it is set to non-zero, the conversion uses aligned_alloc and treats the option value as malloc alignment in order to also use aligned_alloc in relevant cases.


Repository:
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CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D77528/new/

https://reviews.llvm.org/D77528