[llvm-dev] Instruction not in sequence

[Hal Finkel via llvm-dev]

----- 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

> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory