[Bug target/56165] New: Missed optimization for 'noreturn' functions

[akobets at mail dot ru]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56165



             Bug #: 56165

           Summary: Missed optimization for 'noreturn' functions

    Classification: Unclassified

           Product: gcc

           Version: 4.7.2

            Status: UNCONFIRMED

          Severity: normal

          Priority: P3

         Component: target

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: akob...@mail.ru

            Target: x86_64-linux-gnu





Created attachment 29318

  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29318

Example source code



1) Functions with __attribute__((noreturn)) contain unnecessary

push rbx

or

sub rsp,8

instructions in prologue.



2) Tail call need replace to jmp.



Source code (there is 4 highlighted lines with !!!):



===Intrpt64.cpp===

struct _BiosConsole

{

    virtual void  __attribute__((stdcall)) PrintHexDWord(long n);

};



void __attribute__((noreturn)) Intrpt_All(long* thr, long IntNum)

{

    _BiosConsole BiosCons;



    BiosCons.PrintHexDWord((long)thr);

    asm volatile("mov rsp, %0;"

        : : "r" (thr));

    for(;;){}

}



void __attribute__((noreturn)) Intrpt_0()

{

    register long* thr;

    asm volatile("mov %0,rsp" : "=g" (thr));

    Intrpt_All(thr, 0);

}

===END===



Compile:

x86_64-linux-gnu-gcc -c -Wall -Wno-attributes -save-temps -fverbose-asm

-masm=intel -march=core2 -mcmodel=large -mno-mmx -mno-sse -O1 -fno-rtti

-fno-default-inline -fomit-frame-pointer -falign-functions=16

-foptimize-sibling-calls -ffreestanding -fno-stack-protector --no-exceptions

Intrpt64.cpp



Generated Assembly code:



===Intrpt64.s===

    .text

    .p2align 4,,15

    .globl    _Z10Intrpt_AllPll

    .type    _Z10Intrpt_AllPll, @function

_Z10Intrpt_AllPll:

.LFB0:

    .cfi_startproc

    push    rbx    # TRASH !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

    .cfi_def_cfa_offset 16

    .cfi_offset 3, -16

    sub    rsp, 16    #,

    .cfi_def_cfa_offset 32

    mov    rbx, rdi    # thr, thr

    movabs    rax, OFFSET FLAT:_ZTV12_BiosConsole+16    #,

    mov    QWORD PTR [rsp+8], rax    # BiosCons._vptr._BiosConsole,

    mov    rsi, rdi    #, thr

    lea    rdi, [rsp+8]    #,

    movabs    rax, OFFSET FLAT:_ZN12_BiosConsole13PrintHexDWordEl    # tmp64,

    call    rax    # JMP !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

#APP

# 14 "Intrpt64.cpp" 1

    mov rsp, rbx;    # thr

# 0 "" 2

#NO_APP

.L2:

    jmp    .L2    #

    .cfi_endproc

.LFE0:

    .size    _Z10Intrpt_AllPll, .-_Z10Intrpt_AllPll

    .p2align 4,,15

    .globl    _Z8Intrpt_0v

    .type    _Z8Intrpt_0v, @function

_Z8Intrpt_0v:

.LFB4:

    .cfi_startproc

    sub    rsp, 8    # TRASH !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

    .cfi_def_cfa_offset 16

#APP

# 21 "Intrpt64.cpp" 1

    mov rdi,rsp    # thr

# 0 "" 2

#NO_APP

    mov    esi, 0    #,

    movabs    rax, OFFSET FLAT:_Z10Intrpt_AllPll    # tmp61,

    call    rax    # JMP !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

    .cfi_endproc

.LFE4:

    .size    _Z8Intrpt_0v, .-_Z8Intrpt_0v

    .ident    "GCC: (GNU) 4.7.2"

    .section    .note.GNU-stack,"",@progbits

===END===