[LLVMdev] Is it possible to create a variable that aliases a field in a struct ?

[Reid Kleckner]

Previously it was impossible to create an alias with an offset into a
global.  It appears you've found a way to workaround the check in the
verifier, and that understandably crashes LLVM.

Rafael recently relaxed this restriction in trunk LLVM, so aliases can now
be arbitrary constant expressions.  It should support your example.  It is
now possible to represent some aliases in LLVM that cannot be compiled, so
be careful.


On Mon, Jun 16, 2014 at 2:57 AM, sebastien deldon (PGI) <
sebastien.deldon at pgroup.com> wrote:

> Hi all,
>
>
>
> Given assert code in alias detection I’ve opted for a simplification in
> expressing field address :
>
>
>
> 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-v16:16:16-v32:32:32-v64:64:64-v128:128:128-n16:32:64"
>
>
>
>
>
> %struct._dpowk_16 = type < { [32 x [32 x double]], [32 x [32 x double]],
> [32 x [32 x i64]] } >
>
>
>
> @_dpowk_16 = common global %struct._dpowk_16  zeroinitializer, align 16
>
> @r_d_field = alias [32 x [32 x double]] * getelementptr (%struct._dpowk_16
> * @_dpowk_16, i32 0, i32 1)
>
>
>
> Now I’ve got following error from opt :
>
>
>
> opt -O2 -S gblmod2.ll -o gblmod2.opt.ll
>
> opt:
> /home/sdeldon/tools/llvm/3.2/llvm-3.2.src/include/llvm/Support/Casting.h:208:
> typename llvm::cast_retty<To, From>::ret_type llvm::cast(const Y&) [with X
> = llvm::GlobalValue, Y = llvm::Value*]: Assertion `isa<X>(Val) &&
> "cast<Ty>() argument of incompatible type!"' failed.
>
> 0  opt             0x00000000010e3abf
>
> 1  opt             0x00000000010e5a22
>
> 2  libpthread.so.0 0x0000003dd0a0f710
>
> 3  libc.so.6       0x0000003dd0232925 gsignal + 53
>
> 4  libc.so.6       0x0000003dd0234105 abort + 373
>
> 5  libc.so.6       0x0000003dd022ba4e
>
> 6  libc.so.6       0x0000003dd022bb10 __assert_perror_fail + 0
>
> 7  opt             0x0000000000bf238f
>
> 8  opt             0x0000000001083af1
> llvm::MPPassManager::runOnModule(llvm::Module&) + 497
>
> 9  opt             0x0000000001083c6b
> llvm::PassManagerImpl::run(llvm::Module&) + 171
>
> 10 opt             0x000000000053b307 main + 6999
>
> 11 libc.so.6       0x0000003dd021ed1d __libc_start_main + 253
>
> 12 opt             0x000000000052c5c9
>
> Stack dump:
>
> 0.            Program arguments: opt -O2 -S gblmod2.ll -o gblmod2.opt.ll
>
> 1.            Running pass 'Global Variable Optimizer' on module
> 'gblmod2.ll'.
>
>
>
> Is it a BUG ?
>
>
>
> Seb
>
>
>
>
>
> *From:* sebastien deldon (PGI)
> *Sent:* Friday, June 13, 2014 11:05 AM
> *To:* llvmdev
> *Subject:* Is it possible to create a variable that aliases a field in a
> struct ?
>
>
>
> Hi all,
>
>
>
> I would like to create a variable that aliases a field in a struct like :
>
>
>
> %struct._dpowk_16 = type < { [32 x [32 x double]], [32 x [32 x double]],
> [32 x [32 x i64]] } >
>
>
>
> @_dpowk_16 = common addrspace(1) global %struct._dpowk_16
> zeroinitializer, align 16
>
> @r_d_alias = alias [32 x [32 x double]] addrspace(1) * bitcast (i8
> addrspace(1) * getelementptr (i8 addrspace(1) * bitcast (%struct._dpowk_16
> addrspace(1)* @_dpowk_16 to i8 addrspace(1) *), i64 8192) to [32 x [32 x
> double]] addrspace(1) * )
>
>
>
> When I use opt I got following message :
>
>
>
> Aliasee should be either GlobalValue or bitcast of GlobalValue
>
> [32 x [32 x double]] addrspace(1)* @r_d_alias
>
> Broken module found, compilation aborted!
>
> 0  opt             0x00000000010e3abf
>
> 1  opt             0x00000000010e5a22
>
> 2  libpthread.so.0 0x0000003dd0a0f710
>
> 3  libc.so.6       0x0000003dd0232925 gsignal + 53
>
> 4  libc.so.6       0x0000003dd0234105 abort + 373
>
> 5  opt             0x00000000010a3e8a
>
> 6  opt             0x000000000107c654
> llvm::FPPassManager::doFinalization(llvm::Module&) + 84
>
> 7  opt             0x000000000107df79
> llvm::FunctionPassManagerImpl::doFinalization(llvm::Module&) + 105
>
> 8  opt             0x000000000053b25d main + 6829
>
> 9  libc.so.6       0x0000003dd021ed1d __libc_start_main + 253
>
> 10 opt             0x000000000052c5c9
>
> Stack dump:
>
> 0.                Program arguments: opt -O2 -S gblmod.ll -o gblmod.opt.ll
>
>
>
> Is there a way to have a global value that reflects a field in a struct ?
>
>
>
> Thanks for your answers
>
> Seb
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