[cfe-dev] [LLVMdev] A problem with names that can not be demangled.

[Cary Coutant]

GCC has a generalized mangling syntax for cloned functions. See GCC PR 40831:

   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=40831

and the discussion thread here:

   https://gcc.gnu.org/ml/gcc-patches/2011-09/msg01375.html

-cary



On Tue, Apr 14, 2015 at 12:44 PM, David Blaikie <dblaikie at gmail.com> wrote:
> Adding llvm-dev as that might be a more suitable audience for this
> discussion.
>
> (& I know Lang's been playing around with the same problem in the Orc JIT,
> so adding him too)
>
> Is there any basis/reason to believe that the .X suffix is a better, more
> principled one than straight X? Is that documented somewhere as a thing the
> demangling tools will ignore?
>
> On Tue, Apr 14, 2015 at 12:06 PM, Srivastava, Sunil
> <sunil_srivastava at playstation.sony.com> wrote:
>>
>> Hi,
>>
>>
>>
>> We are running into a problem created by renaming of static symbols by
>> llvm-link.  It first
>>
>> showed up using LTO, but we can illustrate this by using llvm-link as
>> well.
>>
>>
>>
>> Say we have two files with the same named static symbol Bye
>>
>>
>>
>> --------------- t1.cpp ---------
>>
>> static void Bye(int* ba1) { ba1[0] /= ba1[2] - 2; }
>>
>> void main_a( int* inB) { void (*func)(int*) = Bye; func(inB); }
>>
>> --------------- t2.cpp ---------
>>
>> static void Bye(int* ba1) { ba1[0] *= ba1[2] + 2; }
>>
>> void main_b( int* inB) { void (*func)(int*) = Bye; func(inB+1); }
>>
>>
>>
>> --------- cmd sequence -------
>>
>> $ clang++ -c -emit-llvm t1.cpp -o t1.bc
>>
>> $ clang++ -c -emit-llvm t1.cpp -o t2.bc
>>
>> $ llvm-link t1.bc t2.bc -o t23.bc
>>
>> $ clang -c t23.bc
>>
>> $ nm t23.o
>>
>>
>>
>> t1.o and t2.o have the same named function “_ZL3ByePi”. In order to
>> distinguish them,
>>
>> one gets a ‘1’ appended to it, making it  “_ZL3ByePi1”.
>>
>>
>>
>> While the code is all correct, the problem is that this modified name
>> cannot be demangled.
>>
>>
>>
>> That is what I am trying to fix.
>>
>>
>>
>> In similar situations gcc appends a ‘.’ before appending the
>> discriminating number, making “_ZL3ByePi.1”
>>
>>
>>
>> The following change in lib/IR/ValueSymbolTable.cpp seems to fix this
>> problem.
>>
>>
>>
>> ------------ start diff -------------------
>>
>> @@ -54,5 +54,5 @@ void ValueSymbolTable::reinsertValue(Value* V) {
>>
>>      // Trim any suffix off and append the next number.
>>
>>      UniqueName.resize(BaseSize);
>>
>> -    raw_svector_ostream(UniqueName) << ++LastUnique;
>>
>> +    raw_svector_ostream(UniqueName) <<  "."  << ++LastUnique;
>>
>>
>>
>>      // Try insert the vmap entry with this suffix.
>>
>> -------------- end diff ---------------------
>>
>>
>>
>> However it causes 60 test failures. These are tests where some names that
>> are expecting
>>
>> to get a plain numeric suffix now have a ‘.’ before it. These are all
>> local symbols, so I think
>>
>> the generated code will always be correct, but the tests as written do not
>> pass. For
>>
>> example, take test/CodeGen/ARM/global-merge-addrspace.ll
>>
>>
>>
>> ; RUN: llc < %s -mtriple=thumb-apple-darwin -O3 | FileCheck %s
>>
>> ; Test the GlobalMerge pass. Check that the pass does not crash when using
>>
>> ; multiple address spaces.
>>
>> ; CHECK: _MergedGlobals:
>>
>> @g1 = internal addrspace(1) global i32 1
>>
>> @g2 = internal addrspace(1) global i32 2
>>
>> ; CHECK: _MergedGlobals1:
>>
>> @g3 = internal addrspace(2) global i32 3
>>
>> @g4 = internal addrspace(2) global i32 4
>>
>>
>>
>> With my change, the symbol is named MergedGlobals.1, hence it fails this
>> test.
>>
>>
>>
>> I could change these 60 tests to match the new behavior. That will fix
>> these 60 failures.
>>
>> However, I do have a concern that there may be other places in llvm that
>> expect the
>>
>> names to be pure identifiers. Adding a ‘.’ may cause them to fail. No such
>> failure has been
>>
>> seen in running the whole clang test, but the concern is still there.
>>
>>
>>
>> I should note that even local symbols are treated similarly, so for
>> example, a parameter
>>
>> named ‘str’ becomes ‘str.1’ with my change, instead of ‘str1’ currently
>> (an actual
>>
>> example from a test).
>>
>>
>>
>> Alternatively, I could try to limit my change to just mangled names.
>>
>>
>>
>> Any suggestion about how this should be fixed ?
>>
>>
>>
>> There is another similar change about 40 lines below in
>> ValueSymbolTable::createValueName().
>>
>> That is not needed to fix this particular problem, but looks similar, so
>> perhaps should be treated
>>
>> similarly for consistency. It causes 66 more failures of the same nature
>> though.
>>
>>
>>
>> Thanks
>>
>>
>>
>> Sunil Srivastava
>>
>> Sony Computer Entertainment
>>
>>
>>
>>
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>>
>
>
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