[llvm-dev] Identify c-style pointer to array

alex via llvm-dev llvm-dev at lists.llvm.org
Tue Jul 25 09:03:48 PDT 2017

> Not usually. C has a rarely used feature to tell the compiler how big
> an array going into a function is:
>     void foo(int arr[static 5]) {}
> With this Clang will add a "dereferenceable(20)" attribute to the
> parameter that does roughly what you're asking. But without that
> you're not allowed to assume anything about the "array" so even
> knowing it was an array is largely pointless at the LLVM level.
> Another alternative if you control the C is to pass a pointer to an array:
>     void foo(int (*arr)[5]) {}
> This will come through to LLVM as "[5 x i32]*" which you can in the type system.
> Cheers.
> Tim.

Hi Tim,

hmm, currently, I'm using the fixed size arrays (as you suggested). This
is fine for self written c code (when we don't need dynamic sized arrays).

However,  c code like
int i=5;
double dArr[i];

in IR translates to a double * of type pointer to double, not aggregate
The last line in IR translates to (vla relates to the is the dynamically
allocated array):
  %arrayidx = getelementptr inbounds double* %vla, i64 3
  store double 4.200000e+01, double* %arrayidx, align 8

which is different to the usual aggregate style call (with leading i32 0):
  %arrayidx = getelementptr inbounds [5 x double]* %0, i32 0, i64 3

I thought the getelemtptr call using 'inbounds' would return a poison
value if the index is out of bounds -> a naive approach would be to
iterate from 0 upwards, try to introduce a gep instruction and stopping
at the first poison value.
This approach however is cumbersome, as I would need to check every
pointer that way to see if it actually relates to an array....


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