[LLVMbugs] [Bug 5959] New: Results obtained from an asm volatile corrupted by optimiser

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=5959

           Summary: Results obtained from an asm volatile corrupted by
                    optimiser
           Product: libraries
           Version: 2.6
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: miscompilation
          Severity: major
          Priority: P2
         Component: Loop Optimizer
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: mark.i.r.muir at gmail.com
                CC: llvmbugs at cs.uiuc.edu


Created an attachment (id=4007)
 --> (http://llvm.org/bugs/attachment.cgi?id=4007)
Test case demonstrating the problem

When inline assembly appears inside a loop, where the result is intentionally
delayed into a later iteration (e.g. to form a shift chain), the optimiser
incorrectly eliminates the delay elements, resulting in corruption.  

This seems to affect all back-ends (the attached example demonstrates it on at
least X86 and PPC). The code pattern exhibiting the bug - a shift chain - is
rather contrived for regular microprocessors, but is an important construct for
highly parallel data-path architectures, for efficiently loading-in streams of
data into registers. They achieve the same thing as the following on a regular
microprocessor architecture:

for (i = 0; i < n; i ++)
    value[i] = stream_get(i);

where stream_get(i) would be inline assembly reading the nth item from the
stream. The shift chain construct allows value[] to be an array of registers,
initialised from a single loop, with only one instance of the inline assembly
in the loop body.

The problem is visible with -O1 from either clang or llvm-gcc, where the loop
remains intact, but the shift chain is optimised away.

With -O2 however, the loop is unrolled, and the program behaves correctly.

See attached source code for an example.

Thanks.


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