[Bug c/58782] New: GCC generates unnecessary _ITM_WU8 for local variables.

["srdjan.stipic at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58782

            Bug ID: 58782
           Summary: GCC generates unnecessary _ITM_WU8 for local
                    variables.
           Product: gcc
           Version: trans-mem
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: srdjan.stipic at gmail dot com

Created attachment 31031
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31031&action=edit
Minimal test case.

GCC generates unnecessary_ITM_WU8 for local variables.
Example:

typedef struct {
  void* firstPtr;
  void* secondPtr;
} pair_t;

__attribute__((transaction_safe)) list_find(void*, void*);

void* list_find_helper (void* listPtr, void* keyPtr) {
  pair_t findPair;

  findPair.firstPtr = keyPtr;   // the _ITM_WU8() is generated here
                                // but is unnecessary because findPair is local

  list_find(listPtr, &findPair);
  return findPair.secondPtr;
}

Compiled with 4.7.3 -O3 -fgnu-tm

Generated code for transactional clone:

_ZGTt16list_find_helperPvS_:
    pushq    %rbx
    movq    %rdi, %rbx
    subq    $16, %rsp
    movq    %rsp, %rdi
    call    _ITM_WU8                  // unnecessary write
    movq    %rsp, %rsi
    movq    %rbx, %rdi
    call    _ZGTt9list_findPvS_
    leaq    8(%rsp), %rdi
    call    _ITM_RU8
    addq    $16, %rsp
    popq    %rbx
    ret

P.S.

This is the example on gcc.godbolt.org:

http://gcc.godbolt.org/#%7B%22version%22%3A3%2C%22filterAsm%22%3A%7B%22labels%22%3Atrue%2C%22directives%22%3Atrue%2C%22commentOnly%22%3Atrue%7D%2C%22compilers%22%3A%5B%7B%22source%22%3A%22typedef%20struct%20%7B%5Cn%20%20void*%20firstPtr%3B%5Cn%20%20void*%20secondPtr%3B%5Cn%7D%20pair_t%3B%5Cn%5Cn__attribute__((transaction_safe))%20void*%20list_find(void*%2C%20void*)%3B%5Cn%5Cn__attribute__((noinline))%5Cnvoid*%20list_find_helper%20(void*%20listPtr%2C%20void*%20keyPtr)%20%7B%5Cn%20%20pair_t%20findPair%3B%5Cn%5Cn%20%20findPair.firstPtr%20%3D%20keyPtr%3B%5Cn%20%20%20%20%5Cn%20%20list_find(listPtr%2C%20%26findPair)%3B%5Cn%20%20return%20findPair.secondPtr%3B%5Cn%7D%5Cn%5Cn%5Cnvoid*%20TMlist_find%20(void*%20hashtablePtr%2C%20void*%20keyPtr)%20%7B%5Cn%20%20void*%20ret%3B%5Cn%20%20__transaction_atomic%20%7B%5Cn%20%20%20%20ret%20%3D%20list_find_helper(hashtablePtr%2C%20keyPtr)%3B%5Cn%20%20%7D%5Cn%20%20return%20ret%3B%5Cn%7D%22%2C%22compiler%22%3A%22%2Fusr%2Fbin%2Fg%2B%2B-4.7%22%2C%22options%22%3A%22-O3%20-fgnu-tm%22%7D%5D%7D