[LLVMdev] Ada bound checks
baldrick at free.fr
Wed May 27 12:41:25 PDT 2009
> Everything worked out with your help. Thanks. I'm now looking at bound
> checks that an Ada program has, so I can remove all possible. It seams
> that Ada already does some optimization to remove bound checks. Do you
> know how are they done, and where?
yes, the front-end doesn't generate range checks in cases where it knows
that everything is ok. It tries hard to catch all simple cases, since
range checks are very expensive. I think this basically means that the
array bounds are known constants (they need not be constant in Ada, a
common example is strings), and the index is too. I think this is done
in ada/checks.adb, search for calls to Compile_Time_Known_Value. Also,
I think the front-end can handle simple symbolic cases like:
for I in A'Range loop ... do something with A(I)... end loop;
By the way, you can see what the front-end is sending to the rest
of gcc by compiling with the -gnatdg flag.
Also, to get all checks required by the language you need to compile
with -gnato. This checks for overflow in all integer computations
(this is off by default because the generated code is so slow).
> I tested two codes (below) and the first code did not produce any bound
Yes, it is too simple. The LLVM optimizers would almost certainly also
have removed the checks.
> and the second produced two bound checks.
You might say four checks, since it does lower and upper bound checks
for each array access.
PS: You might want to take a look at
This is more of a VRP example, but perhaps the kind of pass you have
in mind can help. In theory every call to __gnat_rcheck_12 can be
eliminated, but currently there are 13 calls that llvm fails to remove.
That's mostly because llvm is not informed that Source_First and
Source_Last have values in the range 0 .. 100, and Target_First in the
range 10 .. 20.
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