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From: "zhroma at ispras dot ru" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/55081] New: [4.8 regression?] Non-optimized static array elements initialization
Date: Fri, 26 Oct 2012 13:14:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55081

             Bug #: 55081
           Summary: [4.8 regression?] Non-optimized static array elements
                    initialization
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: zhroma@ispras.ru


Created attachment 28536
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28536
Preprocessed minimized testcase.

In some cases g++ 4.8 revision 192141 and later generate initialization block
for local static array with constant elements, while earlier g++ versions 
insert constants into assembly data section (gcc-base is rev192140, gcc-peak is
rev192141, tested on 64bit Linux):
$ cat test.cpp 
struct R {
        int field;
};
long* foo() {
        R r;
        static long array[] = {
                sizeof(char),
                (reinterpret_cast<long>(&(r.field)) -
reinterpret_cast<long>(&r))+1,
        };
        return array;
}
$ gcc-base/bin/g++ -O2 test.cpp -S -o 1.s
$ gcc-peak/bin/g++ -O2 test.cpp -S -o 2.s 
$ diff -u 1.s 2.s
--- 1.s
+++ 2.s
@@ -6,8 +6,27 @@
 _Z3foov:
 .LFB0:
     .cfi_startproc
+    cmpb    $0, _ZGVZ3foovE5array(%rip)
+    je    .L11
     movl    $_ZZ3foovE5array, %eax
     ret
+    .p2align 4,,10
+    .p2align 3
+.L11:
+    subq    $8, %rsp
+    .cfi_def_cfa_offset 16
+    movl    $_ZGVZ3foovE5array, %edi
+    call    __cxa_guard_acquire
+    testl    %eax, %eax
+    je    .L3
+    movl    $_ZGVZ3foovE5array, %edi
+    movq    $1, _ZZ3foovE5array(%rip)
+    call    __cxa_guard_release
+.L3:
+    movl    $_ZZ3foovE5array, %eax
+    addq    $8, %rsp
+    .cfi_def_cfa_offset 8
+    ret
     .cfi_endproc
 .LFE0:
     .size    _Z3foov, .-_Z3foov
@@ -16,7 +35,9 @@
     .type    _ZZ3foovE5array, @object
     .size    _ZZ3foovE5array, 16
 _ZZ3foovE5array:
+    .zero    8
     .quad    1
-    .quad    1
+    .local    _ZGVZ3foovE5array
+    .comm    _ZGVZ3foovE5array,8,8
     .ident    "GCC: (GNU) 4.8.0 20121005 (experimental)"
     .section    .note.GNU-stack,"",@progbits

So, the value of array[0] (sizeof(char) equals 1) is generated on the first
function call instead of emitting it to assemlby data section directly. If I
remove second constant element

static long array[] = {
                sizeof(char),
        };

.. or reimplement it in the following way

static long array[] = {
                sizeof(char),
                __builtin_offsetof(R, field)+1,
        };

the problem disappears.

As I understand, the patch rev192141 goal is new warning. Maybe it should not
affect codegen so much?

Additional information.
The problem described above lead to Webkit build failure.
There is the following step while generating assembly for Webkit JavaScriptCore
low-level interpreter: it generates dummy executable containing a function with
static array:

static const unsigned extractorTable[308992] = {
unsigned(-1639711386),
(reinterpret_cast<ptrdiff_t>(&(reinterpret_cast<ArrayProfile*>
(0x4000)-unsigned(267773781),
sizeof(ValueProfile),
// and so on...
};

And later this dummy executable file (its data section) is parsed to find all
these sizeof-and-offset values. This certainly seems strange, but when Webkit
is cross-compiled it helps to find offsets without running anything on target.
After gcc revision 192141 that executable-parsing script fails to get all
sizeof(...) values - they are zeros in gcc-generated assembly data section.