[llvm-dev] Clearing the BSS section

[John Criswell via llvm-dev]

On 8/28/15 10:52 AM, devh8h via llvm-dev wrote:
> Hi,
>
> I am writing a function that clears the BSS section on an Cortex-M4 embedded system.

I assume that, for some reason, the operating system is not 
demand-paging in zeroed memory.  Is that correct?

>
> The LLVM (version 3.7.0rc3) code I had wrote  is :
> ;------------
> target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
> target triple = "thumbv7em-none--eabi"
>
> @__bss_start = external global i32
> @__bss_end   = external global i32
>
> define void @clearBSS () nounwind {
> entry:
>    br label %bssLoopTest
>   
> bssLoopTest:
>    %p = phi i32* [@__bss_start, %entry], [%p.next, %bssLoop]
>    %completed = icmp eq i32* %p, @__bss_end
>    br i1 %completed, label %clearCompleted, label %bssLoop
>   
> bssLoop:
>    store i32 0, i32* %p, align 4
>    %p.next = getelementptr inbounds i32, i32* %p, i32 1
>    br label %bssLoopTest
>   
> clearCompleted:
>    ret void
> }
> ;------------
>
> This code runs. But when I optimize it with :
> 	opt -disable-simplify-libcalls -Os -S source.ll -o optimized.ll
>
> I get the following code for the @clearBSS function :
> ;------------
> define void @clearBSS() nounwind {
> entry:
>    br label %bssLoop
>
> bssLoop:                                          ; preds = %entry, %bssLoop
>    %p1 = phi i32* [ @__bss_start, %entry ], [ %p.next, %bssLoop ]
>    store i32 0, i32* %p1, align 4
>    %p.next = getelementptr inbounds i32, i32* %p1, i32 1
>    %completed = icmp eq i32* %p.next, @__bss_end
>    br i1 %completed, label %clearCompleted, label %bssLoop
>
> clearCompleted:                                   ; preds = %bssLoop
>    ret void
> }
> ;------------
> The optimizer has transformed the while loop into a repeat until.
>
> I think it assumes the two variables @__bss_start and @__bss_end are distinct. But they are solved at link time, and they are the same if the BSS section is empty : in this case, the optimized function fails.
>
> Is there a way to prevent the optimizer to assume the two variables are distinct ? Or what is the proper way to deal with link time values ?

Have you tried using the memset intrinsic?  You could case bss_start and 
bss_end to integers, subtract them to find the length, and then use 
memset to zero the memory.  I would think memset should work if the 
length is zero.

Regards,

John Criswell

>
> Thanks,
>
> Pierre Molinaro
>
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-- 
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell