[LLVMbugs] [Bug 16248] New: movaps used for unaligned memory involving va_list and nested structs

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Thu Jun 6 14:01:37 PDT 2013


            Bug ID: 16248
           Summary: movaps used for unaligned memory involving va_list and
                    nested structs
           Product: clang
           Version: 3.2
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: llvm-bugs at justinbogner.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The following produces movaps instrcutions for the access of Uuid128_t
from the va_list on x86_64:

    /* movaps.c - compile as "clang -O2 movaps.c -S -o movaps.s" */
    #include <stdarg.h>

    typedef struct Uuid128 {
        __uint128_t __uint;
    } Uuid128_t;

    typedef struct SiteUuid {
      Uuid128_t   su_uuid;
    } SiteUuid_t;

    loadSiteUuid(void *entryRef, va_list argList)
        SiteUuid_t *su = entryRef;

        su->su_uuid = va_arg(argList, Uuid128_t);

When called, this tends to crash, as the __uint128_t doesn't seem to
end up at a 16 byte aligned address.

On the other hand, this very similar program generates four mov
instructions instead, thus working correctly:

    /* 4mov.c - compile as "clang -O2 4mov.c -S -o 4mov.s" */
    #include <stdarg.h>

    typedef struct Uuid128 {
        __uint128_t __uint;
    } Uuid128_t;

    loadSiteUuid(void *entryRef, va_list argList)
        Uuid128_t *su = entryRef;

        su->su_uuid = va_arg(argList, __uint128_t);

This makes me think that clang recognizes the alignment requirements
of __uint128_t and avoid the vector op, but that nesting it in a
struct confuses things.

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