[llvm-dev] Clearing the BSS section

[devh8h via llvm-dev]

Hi,

I am writing a function that clears the BSS section on an Cortex-M4 embedded system.

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 ?

Thanks,

Pierre Molinaro