[PATCH] D40573: [NVPTX] Assign valid global names

[Jonas Hahnfeld via Phabricator via llvm-commits]

Hahnfeld added inline comments.


================
Comment at: lib/IR/ValueSymbolTable.cpp:54-59
+      // MSVC name mangling but is fine for PTX.
+      if (M && Triple(M->getTargetTriple()).isNVPTX())
+        S << "$";
+      else
+        S << ".";
+    }
----------------
tra wrote:
> Hahnfeld wrote:
> > tra wrote:
> > > Hahnfeld wrote:
> > > > tra wrote:
> > > > > This patch addresses "we can't compile generated PTX because LLVM uses illegal characters", but exposes another issue -- having potentially different names on host and device is a problem for CUDA. For some objects host side may need to know what it's called on device side. We need it in order to access it from host (eg cudaMemcpyToSymbol(), or initializing static variables) and we currently assume that the names are the same. If such symbol gets different names on host and device, compilation will succeed, but we'll have problems at runtime.
> > > > > 
> > > > > Does "." have any special meaning? Can we skip the unique delimiter altogether? 
> > > > > 
> > > > > If we can't find a suitable way to guarantee identical naming, we'll need a way to have a reliable way to determine the name used on the other side of the compilation.
> > > > > 
> > > > > 
> > > > So the interesting question is: When will this code ever be hit? Most programming languages (including C and C++) obviously don't allow multiple variables of the same name - how would the compiler say which symbol you meant. The use case I've mostly seen is for compiler generated function, `omp_outlined` for example. These can be generated multiple times in the same translation unit and have to get unique names. Do you have another example where this could happen?
> > > > 
> > > > I'm not really sure '.' has a special meaning. Maybe @rafael can help because one of his old commits (https://reviews.llvm.org/rL253804) says `For globals it is important to use "foo.1" to help C++ name demangling.`
> > > I vaguely recall that '.' was an indication for demangler that it should not proceed further. I.e. a sort-of-special character to indicate the end of the C++-mangled part of the symbol name.
> > > 
> > > If name mangling can't be made identical (and it looks like it may be the case), we can probably work around it. I.e. for symbols that must have identical names on both sides we can generate a unique alias that's identical on both sides and use it instead when CUDA needs it.
> > > 
> > But why is LLVM responsible for mangling? Shouldn't this be done by the Clang frontend?
> > I've found this test in `libcxxabi/test/test_demangle.pass.cpp`:
> > ```
> > {"_ZNK10__cxxabiv111__libcxxabi5__sub20first_demangled_nameEPc.eh", "__cxxabiv1::__libcxxabi::__sub::first_demangled_name(char*) const (.eh)"},
> > ```
> > 
> > As said, I can't imagine a user defined value where LLVM needs to generate a unique name, so this should be fine for the CUDA functions you mentioned because they operate on variables.
> Sorry. I should've said "the way LLVM creates unique names".  C++ mangling is indeed handled by clang.
> 
> AFAICT, we do need to consider possibility of LLVM generating unique name whenever  Clang calls LLVM's Value::setName(). The API explicitly guarantees that the name will be unique.
> 
> Another case would be an identifier with a unicode symbol in it. NVPTX would have to sanitize it with the result potentially clashing with existing name -- similar to @.str test case below. This is hypothetical at the moment as clang does not support unicode (yet?).
> 
> 
All right, then the next natural question is: Do we need to keep the C++ mangling in mind when creating the unique name? Because that is currently the reason that there is a dot separator. If we don't need it we could just drop the dot and every target would be happy. In addition this would guarantee that LLVM will produce a "valid" name after NVPTX sanitized the name and there was a crash.


https://reviews.llvm.org/D40573