[llvm-dev] Question about Constant expressions

[Johannes Doerfert via llvm-dev]

Because in the first case the GEP is an instruction and in the second 
case a Constant.

We have helpers, like GEPOperand, to handle both the same way.


On 6/13/20 10:29 AM, James Courtier-Dutton wrote:
> Hi,
>
> What I meant was, why is it not represented by something like this instead:
>
> define dso_local i32 @test21() #0 {
>    %1 = i8* getelementptr inbounds ([8 x
> i8], [8 x i8]* @.str, i64 0, i64 0)
>    %2 = call i32 (i8*, ...) @printf(%1)
>    ret i32 0
> }
>
> instead of:
>
> define dso_local i32 @test21() #0 {
>    %1 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([8 x
> i8], [8 x i8]* @.str, i64 0, i64 0))
>    ret i32 0
> }
>
> Some code I am writing would be a lot simpler if the first case was used.
>
>
> On Sun, 31 May 2020 at 16:56, Johannes Doerfert
> <johannesdoerfert at gmail.com> wrote:
>> Hi James,
>>
>>
>> Like other constant expressions, e.g., i32 0, this GEP constant
>>
>> expression are "shown" inlined. Constants are unique and exist
>>
>> everywhere. I guess the most "important' use case I can think
>>
>> of is to use such GEPs as part of other constant expressions,
>>
>> e.g., initializers of constant globals.
>>
>>
>> Cheers,
>>
>>    Johannes
>>
>>
>> On 5/31/20 3:49 AM, James Courtier-Dutton via llvm-dev wrote:
>>
>> Hi,
>>
>> Why is the getelementptr part of the call instruction, instead of
>> being a separate instruction in the LLVM IR ?
>> The LLVM IR Builder adds it as a separate instruction, but it is
>> rolled into one call instruction in the .bc output.
>> I am just curious as to why it is done this way.
>>
>> #include <stdint.h>
>> #include <stdio.h>
>> int test21(void);
>> int test21(void) {
>>          printf("Hello1\n");
>>          return 0;
>> }
>>
>> define dso_local i32 @test21() #0 {
>>    %1 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([8 x
>> i8], [8 x i8]* @.str, i64 0, i64 0))
>>    ret i32 0
>> }
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