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

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

[srdjan.stipic at gmail dot com]

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

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

[jakub at gcc dot gnu.org]

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

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
It is local, but address taken, and the address may escape to other threads. 
In this testcase that isn't a problem, because the store happens before the
address is taken, but that would require extra analysis - address taken
automatic variables are not in SSA form and the flag that it is addressable is
just a global flag.

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

[srdjan.stipic at gmail dot com]

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

--- Comment #2 from Srdjan Stipic <srdjan.stipic at gmail dot com> ---

Hi Jakub.

> In this testcase that isn't a problem, because the store happens before the
> address is taken, but that would require extra analysis

This is the reason why I submitted the bug.
I expected that gcc would eliminate unnecessary writes
until the address is taken.

Here is the case when the address is not taken but gcc generates
unnecessary _ITM_WU4():

void* f(int x) {
  int* out;
  out = (int*)malloc(sizeof(int));
  *out = x;
  return out;
}

_ZGTt1fi:
    movq    %rbx, -16(%rsp)
    movq    %rbp, -8(%rsp)
    subq    $24, %rsp
    movl    %edi, %ebp
    movl    $4, %edi
    call    _ITM_malloc
    movl    %ebp, %esi
    movq    %rax, %rbx
    movq    %rax, %rdi
    call    _ITM_WU4
    movq    %rbx, %rax
    movq    16(%rsp), %rbp
    movq    8(%rsp), %rbx
    addq    $24, %rsp
    ret

Why gcc generates _ITM_WU4 when the address is not taken?
(I can see that 'out' escapes)

Cheers,
Srdjan