[llvm-dev] Instruction not in sequence

[Russell Wallace via llvm-dev]

Oh! Now I see, the thing that printed as getelementptr is as far as the
inheritance hierarchy is concerned quite a different thing, it's a gep
ConstantExpr rather than a gep Instruction. That makes sense, thanks!

On Sun, Nov 8, 2015 at 10:31 AM, Russell Wallace <russell.wallace at gmail.com>
wrote:

> Ah! Okay, that makes sense except for one thing.
>
> The getelementptr successfully dyn_cast<Constant>'ed. At first glance that
> figures since it is a constant expression. But from the inheritance diagram
> of Value, getelementptr is not a subclass of Constant but of Instruction;
> from that perspective I would expect that the fact that this particular one
> is a constant expression would be as far as the compiler is concerned just
> a dynamic fact based on the fact that its operands happen to be constants,
> so I wouldn't have expected the dynamic cast to succeed. What am I missing
> here?
>
> On Sun, Nov 8, 2015 at 10:10 AM, Hal Finkel <hfinkel at anl.gov> wrote:
>
>> ----- Original Message -----
>> > From: "Russell Wallace via llvm-dev" <llvm-dev at lists.llvm.org>
>> > To: "llvm-dev" <llvm-dev at lists.llvm.org>
>> > Sent: Sunday, November 8, 2015 4:03:27 AM
>> > Subject: [llvm-dev] Instruction not in sequence
>> >
>> > This code:
>> >
>> > int main(int argc, char **argv) {
>> > puts("abc");
>> > return 0;
>> > }
>> >
>> > compiles to this:
>> >
>> > @"\01??_C at _03FIKCJHKP@abc?$AA@" = linkonce_odr unnamed_addr constant
>> > [4 x i8] c"abc\00", comdat, align 1
>> >
>> > ; Function Attrs: nounwind uwtable
>> > define i32 @main(i32 %argc, i8** nocapture readnone %argv) #0 {
>> > %1 = tail call i32 @puts(i8* getelementptr inbounds ([4 x i8], [4 x
>> > i8]* @"\01??_C at _03FIKCJHKP@abc?$AA@", i64 0, i64 0)) #2, !dbg !15
>> > ret i32 0, !dbg !16
>> > }
>> >
>> > The part I don't understand is where the call instruction refers to
>> > the result of getelementptr without the latter appearing in the
>> > sequence of instructions. I thought all instructions appeared in
>> > sequence in basic blocks, so I was expecting %1 = getelementptr ...,
>> > %2 = call(%1 ...)
>> >
>> > Is there something I'm missing about the rules for the intermediate
>> > code representation?
>>
>> The getelementptr here is not an instruction at all, but rather a
>> constant expression. Constant expressions in LLVM can be complicated, and
>> have expression nodes that mirror the usual IR instructions.
>>
>> A quick way to see this is to run your IR through llc -march=cpp.
>>
>>  -Hal
>>
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
>
>
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