https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79349
Bug ID: 79349
Summary: unused std::string is not optimized away in presense
of a call
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: hiraditya at msn dot com
Target Milestone: ---
g++ version (GCC) 7.0.0 20170118 (experimental)
$ cat t.cpp
#include<string>
void foo();
int main() {
std::string s("abc");
foo ();
return 0;
}
$ install/bin/g++ -O3 t.cpp -S -o t.s
$ cat t.s
main:
.LFB995:
.cfi_startproc
.cfi_personality 0x3,__gxx_personality_v0
.cfi_lsda 0x3,.LLSDA995
pushq %rbx
.cfi_def_cfa_offset 16
.cfi_offset 3, -16
subq $32, %rsp
.cfi_def_cfa_offset 48
leaq 16(%rsp), %rax
movb $99, 18(%rsp)
movq $3, 8(%rsp)
movb $0, 19(%rsp)
movq %rax, (%rsp)
movl $25185, %eax
movw %ax, 16(%rsp)
.LEHB0:
call _Z3foov
.LEHE0:
movq (%rsp), %rdi
leaq 16(%rsp), %rax
cmpq %rax, %rdi
je .L6
call _ZdlPv
.L6:
addq $32, %rsp
.cfi_remember_state
.cfi_def_cfa_offset 16
xorl %eax, %eax
popq %rbx
.cfi_def_cfa_offset 8
ret
.L5:
.cfi_restore_state
movq (%rsp), %rdi
leaq 16(%rsp), %rdx
movq %rax, %rbx
cmpq %rdx, %rdi
je .L4
call _ZdlPv
.L4:
movq %rbx, %rdi
.LEHB1:
call _Unwind_Resume
.LEHE1:
.cfi_endproc
.LFE995:
.globl __gxx_personality_v0
.section .gcc_except_table,"a",@progbits
While clang++ optimizes it away: clang version 5.0.0 (llvm-project SHA:
28b7c19c2379e17b26571260933467b9f98b449c)
$ ./bin/clang++ -O3 t.cpp -S -o t.s -stdlib=libc++
$ cat t.s
main: # @main
.cfi_startproc
# BB#0: # %entry
pushq %rax
.Lcfi0:
.cfi_def_cfa_offset 16
callq _Z3foov
xorl %eax, %eax
popq %rcx
retq
.Lfunc_end0:
.size main, .Lfunc_end0-main
.cfi_endproc
.ident "clang version 5.0.0 "
.section ".note.GNU-stack","",@progbits
