llvm dev - Jul 2019 - Accessing global variables arrays result in inlined getelementptrs

Hello,

Using LLVM+clang to do some static analysis, I face some issues due to 
the fact that when manipulating arrays that are global variables, IR 
getelementptr instructions seem to be somehow inlined.

For example:

int a[256];
int main() {
     a[0] = 1;
}

gives as a result the following IR code (with clang+llvm 8, without any 
flag).

@a = common dso_local global [256 x i32] zeroinitializer, align 16

; Function Attrs: noinline nounwind optnone sspstrong uwtable
define dso_local i32 @main() #0 {
   store i32 1, i32* getelementptr inbounds ([256 x i32], [256 x i32]* 
@a, i64 0, i64 0), align 16
   ret i32 0
}

Hence two questions.
- What is the main reason behind this behavior?
- Is there a way to disable it (ideally while preserving optimisations 
disabled)?

Thank you in advance,
Antonin Reitz

Am Di., 23. Juli 2019 um 15:11 Uhr schrieb Antonin Reitz via llvm-dev
<llvm-dev at lists.llvm.org>:
> Hence two questions.
> - What is the main reason behind this behavior?
The getelementptr has all constant argument, meaning it itself becomes
an GetElementPtrConstantExpr. ConstantExprs (in contrast to
instructions) are inlined by the IR emitter.

> - Is there a way to disable it (ideally while preserving optimisations
> disabled)?
Not that I know of.

Michael

On 7/23/19 3:38 PM, Michael Kruse wrote:
> The getelementptr has all constant argument, meaning it itself becomes
> an GetElementPtrConstantExpr. ConstantExprs (in contrast to
> instructions) are inlined by the IR emitter.
>> - Is there a way to disable it (ideally while preserving optimisations
>> disabled)?
Is it something that would be relatively easy to change with a local 
patch for LLVM without introducing a lot of modifications? (only 
disabling GetElementPtrConstantExprs inlining)

This is really something that would help having simpler static analysis, 
even if additional analysis of the code can be used as a (quite ugly) 
workaround.

Thank you very much,

Antonin