Re: [Qemu-devel] [PATCH v3 04/18] build-sys: compile with -Og or -O1 when --enable-debug

[Philippe Mathieu-Daudé]

On 01/04/2018 01:05 PM, Marc-André Lureau wrote:
> When --enable-debug is turned on, configure doesn't set -O level, and
> uses default compiler -O0 level, which is slow.
> 
> Instead, use -Og if supported by the compiler (optimize debugging
> experience), or -O1 (keeps code somewhat debuggable and works around
> compiler bugs).
> 
> Unfortunately, gcc has many false-positive maybe-uninitialized
> errors with Og and O1 (f27 gcc 7.2.1 20170915):
> 
> /home/elmarco/src/qemu/hw/ipmi/isa_ipmi_kcs.c: In function 
> ‘ipmi_kcs_ioport_read’:
> /home/elmarco/src/qemu/hw/ipmi/isa_ipmi_kcs.c:279:12: error: ‘ret’ may be 
> used uninitialized in this function [-Werror=maybe-uninitialized]
>      return ret;
>             ^~~
> cc1: all warnings being treated as errors
> make: *** [/home/elmarco/src/qemu/rules.mak:66: hw/ipmi/isa_ipmi_kcs.o] Error 
> 1
> make: *** Waiting for unfinished jobs....
> /home/elmarco/src/qemu/hw/ide/ahci.c: In function ‘ahci_populate_sglist’:
> /home/elmarco/src/qemu/hw/ide/ahci.c:903:58: error: ‘tbl_entry_size’ may be 
> used uninitialized in this function [-Werror=maybe-uninitialized]
>          if ((off_idx == -1) || (off_pos < 0) || (off_pos > tbl_entry_size)) {
>                                                  ~~~~~~~~~^~~~~~~~~~~~~~~~~
> cc1: all warnings being treated as errors
> make: *** [/home/elmarco/src/qemu/rules.mak:66: hw/ide/ahci.o] Error 1
> /home/elmarco/src/qemu/hw/display/qxl.c: In function ‘qxl_add_memslot’:
> /home/elmarco/src/qemu/hw/display/qxl.c:1397:52: error: ‘pci_start’ may be 
> used uninitialized in this function [-Werror=maybe-uninitialized]
>      memslot.virt_end   = virt_start + (guest_end   - pci_start);
>                                        ~~~~~~~~~~~~~^~~~~~~~~~~~
> /home/elmarco/src/qemu/hw/display/qxl.c:1389:9: error: ‘pci_region’ may be 
> used uninitialized in this function [-Werror=maybe-uninitialized]
>          qxl_set_guest_bug(d, "%s: pci_region = %d", __func__, pci_region);
>          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> cc1: all warnings being treated as errors
> 
> There seems to be a long list of related bugs in upstream GCC, some of
> them are being fixed very recently:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24639
> 
> For now, let's workaround it by using Wno-maybe-uninitialized (gcc-only).
> 
> Suggested-by: Paolo Bonzini <address@hidden>
> Signed-off-by: Marc-André Lureau <address@hidden>

Tested-by: Philippe Mathieu-Daudé <address@hidden>

> ---
>  configure | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/configure b/configure
> index de1f939a28..3953859314 100755
> --- a/configure
> +++ b/configure
> @@ -5160,8 +5160,19 @@ if test "$gcov" = "yes" ; then
>    LDFLAGS="-fprofile-arcs -ftest-coverage $LDFLAGS"
>  elif test "$fortify_source" = "yes" ; then
>    CFLAGS="-O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 $CFLAGS"
> -elif test "$debug" = "no"; then
> -  CFLAGS="-O2 $CFLAGS"
> +elif test "$debug" = "yes"; then
> +  if compile_prog "-Og" ""; then
> +      CFLAGS="-Og $CFLAGS"
> +  elif compile_prog "-O1" ""; then
> +      CFLAGS="-O1 $CFLAGS"
> +  fi
> +  # Workaround GCC false-positive Wuninitialized bugs with Og or O1:
> +  # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24639
> +  if cc_has_warning_flag "-Wno-maybe-uninitialized"; then
> +      CFLAGS="-Wno-maybe-uninitialized $CFLAGS"
> +  fi
> +else
> +    CFLAGS="-O2 $CFLAGS"
>  fi
>  
>  ##########################################
>