Help on finding Base GEP

Project Infrastructure LLVM Dev List Archives
#1

Hi,

Below is the snippet of LLVM IR code generated by Flang

—snip–
%3 = getelementptr i64, i64* %“a$sd”, i64 11, !dbg !16

%4 = bitcast i64* %3 to i32*, !dbg !16
%5 = load i32, i32* %4, align 4, !dbg !16, !tbaa !22

—snip–

My requirement is for any such LoadInst (example: %5), I want to check if its base GEP (i.e. %3) loads at particular offset (11) from its corresponding arrays descriptor structure (a$sd).

is there any API in LLVM to check this? or I need to write my own?

regards,
Venkat.

#2

You can run SCEV which will see through bitcasts and GEPs, so it should give an expression of the form (base + offset) on the pointer of the load.

Cheers
siddharth

#3

Specifically, you can use ScalarEvolution to subtract the expression for the base descriptor from the expression for the loaded pointer and see if the result is a SCEVConstant. Also, you might find GetPointerBaseWithConstantOffset (in ValueTracking.h) useful. -Hal

#4

Hi,