[LLVMbugs] [Bug 8388] New: memcpy/memmove/memset intrinsics are unclear about alignment value 0

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Fri Oct 15 12:45:03 PDT 2010


           Summary: memcpy/memmove/memset intrinsics are unclear about
                    alignment value 0
           Product: Documentation
           Version: trunk
          Platform: All
               URL: http://llvm.org/docs/LangRef.html#int_memcpy
        OS/Version: All
            Status: NEW
          Keywords: documentation
          Severity: normal
          Priority: P
         Component: General docs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: matti.niemenmaa+llvmbugs at iki.fi
                CC: llvmbugs at cs.uiuc.edu

The docs for llvm.memcpy.*, llvm.memmove.*, and llvm.memset.* say the following
about alignment value 0:

  "If the call to this intrinsic has an alignment value that is not 0 or 1,
then the caller guarantees that both the source and destination pointers are
aligned to that boundary."

  "If the argument is known to be aligned to some boundary, this can be
specified as the fourth argument, otherwise it should be set to 0 or 1."

This seems to imply that an alignment value of 0 has the same effect as 1. At
best, it doesn't say what 0 means.

For the 'load' and 'store' instructions, the docs say that a value of 0 allows
the backend to assume the best alignment for the target. This seems to be the
actual behaviour for the llvm.mem*.* intrinsics as well. For example, the
memset in the following gets codegenned to a 'movaps', causing a segfault with
llc -O0 (-O1 and above get lucky with the choice of stack slot):

target datalayout =
target triple = "x86_64-unknown-linux-gnu"
define i32 @main() {
  %a = alloca [4 x i32], align 1
  %p = bitcast [4 x i32]* %a to i8*
  call void @llvm.memset.p0i8.i64(i8* %p, i8 0, i64 16, i32 0, i1 false)
  ret i32 0
declare void @llvm.memset.p0i8.i64(i8* nocapture, i8, i64, i32, i1) nounwind

The docs could use some clarification and say that if the alignment value is 0,
it's treated as meaning arbitrary alignment.

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