[cfe-commits] r147730 - in /cfe/trunk: include/clang/Sema/Sema.h lib/CodeGen/CodeGenFunction.cpp lib/Parse/ParseDecl.cpp lib/Sema/SemaDecl.cpp test/CodeGen/vla.c

[Abramo Bagnara]

Il 08/01/2012 09:35, Eli Friedman ha scritto:
> On Sun, Jan 8, 2012 at 12:12 AM, Abramo Bagnara
> <abramo.bagnara at gmail.com> wrote:
>> $ cat p.cc
>> long f();
>> long g();
>>
>> void m() {
>>  __decltype((int (*)[f()])g()) x;
>> }
>>
>> $ g++ -O2 -S p.cc -o -
>>        .file   "p.cc"
>>        .text
>>        .p2align 4,,15
>>        .globl  _Z1mv
>>        .type   _Z1mv, @function
>> _Z1mv:
>> .LFB0:
>>        .cfi_startproc
>>        rep
>>        ret
>>        .cfi_endproc
>> .LFE0:
>>        .size   _Z1mv, .-_Z1mv
>>        .ident  "GCC: (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1"
>>        .section        .note.GNU-stack,"", at progbits
> 
> That's... interesting... what happens if you try to return sizeof(*x)?

It calls f() (but not g()), just like clang does. The difference between
the two compilers is that clang always generates the call to f while gcc
generates it only when needed.

However neither of them evaluates the decltype expression but only the
vla type size. This can be verified by the missing call to g.

$ cat p.cc
long f();
long g();

long m() {
  __decltype((int (*)[f()])g()) x;
  return sizeof(*x);
}

$ g++ -O2 -S p.cc -o -
	.file	"p.cc"
	.text
	.p2align 4,,15
	.globl	_Z1mv
	.type	_Z1mv, @function
_Z1mv:
.LFB0:
	.cfi_startproc
	subq	$8, %rsp
	.cfi_def_cfa_offset 16
	call	_Z1fv
	addq	$8, %rsp
	.cfi_def_cfa_offset 8
	salq	$2, %rax
	ret
	.cfi_endproc
.LFE0:
	.size	_Z1mv, .-_Z1mv
	.ident	"GCC: (Ubuntu/Linaro 4.6.1-9ubuntu3) 4.6.1"
	.section	.note.GNU-stack,"", at progbits