[LLVMbugs] [Bug 5599] New: clang doesn't follow x86-64 ABI array alignment rule

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=5599

           Summary: clang doesn't follow x86-64 ABI array alignment rule
           Product: clang
           Version: trunk
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: LLVM Codegen
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: gohman at apple.com
                CC: llvmbugs at cs.uiuc.edu


The x86-64 ABI, in section 3.1.2 Data Representation, subsection "Aggregates
and Unions", second paragraph, says:

"An array uses the same alignment as its elements, except that a local or
global array variable of length at least 16 bytes or a C99 variable-length
array
variable always has alignment of at least 16 bytes."

For the following testcase:

void frob(void *);

void foo(void) {
  float x[4];
  frob(x);
}

clang -O2 -emit-llvm -m64 emits

  %x = alloca [4 x float], align 4                ; <[4 x float]*> [#uses=1]

which shows that the array is not being aligned on a 16 byte boundary.


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