[LLVMdev] load/store in IR without stack/heap

John Criswell criswell at illinois.edu
Thu Jul 7 10:26:07 PDT 2011

On 7/5/11 1:12 PM, Sungjin Kim wrote:
> Hi all,
> Can anyone give an idea to solve my problem? I'm implementing backend 
> part
> using LLVM for my research architecture. The main issue is that this 
> architecture
> cannot use stack/heap. So, all the value should be stored in the register.
> Given that architecture, load/store instruction in IR uses virtual 
> register to load/
> store the value. For example:

It's difficult to answer this question without understanding your 
architecture's replacement for the stack and heap.  While variables like 
those below can be moved into LLVM virtual registers using -mem2reg, not 
all variables can be promoted in such a fashion (for example, an 
address-taken local variable cannot be promoted to a register).  Also, 
how does your architecture support structures and arrays?  Does it 
support global variables?  Is it designed to support programs written in 
languages such as C (in which the stack and heap are assumed to exist)?

There are some things that you can do to promote stack objects to global 
objects.  If a call graph analysis (like the one found in LLVM or the 
one we provide in DSA) indicates that a function is not called 
recursively (i.e., it is not part of a strongly connected component in 
the call graph), then you should be able to replace the stack 
allocations within the function with a global variables (provided that 
the allocas are not in loops).  I think you can do the same with heap 
allocations provided that they don't escape the function in which they 
are allocated.

-- John T.

> C source code is:
> if(...) {
>     a = 1;
> }
> else {
>     a = 0;
> }
> c = a;
> It's IR from the front-end is:
> ...
> ;<label>:3
> store i16 1, i16 *a, align 2
> br label %5
> ;<label>:4
> store i16 0, i16 *a, align 2
> br label %5
> ;<label>:5
> %6 = load i16 *a, align 2
> store i16 %6, i16 c
> I used getCopyToReg in SelectionDAG for store instruction to store 
> value, and getCopyFromReg
> for load instruction. So, storage values in block '<label>:3' and 
> '<label>:4' are stored in VR0 and
> VR1 respectively. However, load instruction in block '<label>:5' 
> cannot choose which register
> should be read.
> Can anybody give an idea to overcome such a case?
> Thanks.
> Jin
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