[llvm-dev] Clearing the BSS section

[devh8h via llvm-dev]

I had thought to use the memset intrinsic, unfortunately I did not succeed to cross compiling compiler-rt on my Mac.

Regards,

Pierre Molinaro



> Le 28 août 2015 à 17:22, John Criswell <jtcriswel at gmail.com> a écrit :
> 
> On 8/28/15 11:20 AM, devh8h wrote:
>> It is a very basic "blink-led" program, on a Teensy 3.1. There is no operating system. The BSS clear function is called at boot. I would write a general BSS clear function, that behaves correctly even if the BSS section is empty.
> 
> I thought it was something like that.
> 
> Let me know if the memset intrinsic approach works.
> 
> Regards,
> 
> John Criswell
> 
>> 
>> Thank,
>> 
>> Pierre Molinaro
>> 
>>> Le 28 août 2015 à 17:00, John Criswell <jtcriswel at gmail.com> a écrit :
>>> 
>>> 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
>>>> 
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>> 
>>> -- 
>>> John Criswell
>>> Assistant Professor
>>> Department of Computer Science, University of Rochester
>>> http://www.cs.rochester.edu/u/criswell
>>> 
> 
> 
> -- 
> John Criswell
> Assistant Professor
> Department of Computer Science, University of Rochester
> http://www.cs.rochester.edu/u/criswell
> 

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