On 15/11/2023 17:08, Tamar Christina wrote:
Hi All,
At the moment we emit a warning whenever you specify both -march and -mcpu
and the architecture of them differ. The idea originally was that the user may
not be aware of this change.
However this has a few problems:
1. Architecture revisions is not an observable part of the architecture,
extensions are. Starting with GCC 14 we have therefore relaxed the rule
that
all extensions can be enabled at any architecture level. Therefore it's
incorrect, or at least not useful to keep the check on architecture.
2. It's problematic in Makefiles and other build systems, where you want to
for certain files enable CPU specific builds. i.e. you may be by default
building for -march=armv8-a but for some file for -mcpu=neoverse-n1. Since
there's no easy way to remove the earlier options we end up warning and
there's no way to disable just this warning. Build systems compiling with
-Werror face an issue in this case that compiling with GCC is needlessly
hard.
3. It doesn't actually warn for cases that may lead to issues, so e.g.
-march=armv8.2-a+sve -mcpu=neoverse-n1 does not give a warning that SVE
would
be disabled.
For this reason I have one of two proposals:
1. Just remove this warning all together.
2. Rework the warning based on extensions and only warn when features would be
disabled by the presence of the -mcpu. This is the approach this patch has
taken.
As examples:
aarch64-none-linux-gnu-gcc -march=armv8.2-a+sve -mcpu=neoverse-n1
cc1: warning: switch ‘-mcpu=neoverse-n1’ conflicts with ‘-march=armv8.2-a+sve’
switch and resulted in options +crc+sve+norcpc+nodotprod being added
.arch armv8.2-a+crc+sve
aarch64-none-linux-gnu-gcc -march=armv8.2-a -mcpu=neoverse-n1
aarch64-none-linux-gnu-gcc -march=armv8.2-a+dotprod -mcpu=neoverse-n1
aarch64-none-linux-gnu-gcc -march=armv8.2-a+dotprod -mcpu=neoverse-n2
<no warning>
The one remaining issue here is that if both -march and -mcpu are specified we
pick the -march. This is not particularly obvious and for the use case to be
more useful I think it makes sense to pick the CPU's arch?
The intent was always that users would either specify -march (with
-mtune) or they would specify just -mcpu, not that they should mix both
on the the command line. -mcpu=<cpu> is supposed to be the equivalent
of -march=<arch-of(cpu)> -mtune=<cpu>. Both the Arm and AArch64
compilers implement the rule that -march dominates any -mcpu setting and
this is (or at least used to be) documented in the manual.
Part of the problem is that there's no clear way to recover positional
information from the parameter list, so that it's not possible in the
specs files to determine whether the user wrote
-mcpu=x -march=y
or
-march=y -mcpu=x
Now if a single source of rules for the cpu/arch conflates things in
this way, that is pilot error, but we can't currently distinguish that
case from the one where, say, the user adds -mcpu to CFLAGS in a
makefile, but the make rules themselves need specific architecture
features in order to build a specific file.
This is where unset becomes useful as it will provide a clean(er) way to
say ignore any conflict from an earlier option and use the following flags.
Hence
(-mcpu=x) (-mcpu=unset -march=y)
(parenthesis indicate different sources of flags) will cause the
compiler to forget any earlier -mcpu and just look at -march. Conversely,
(-march=y) (-march=unset -mcpu=x)
would clear any earlier -march flag and tell the compiler to just use
-mcpu from now on.
I did not make that change in the patch as it changes semantics.
Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
Note that I can't write a test for this because dg-warning expects warnings to
be at a particular line and doesn't support warnings at the "global" level.
Ok for master?
Thanks,
Tamar
gcc/ChangeLog:
* config/aarch64/aarch64.cc (aarch64_override_options): Rework warnings.
--- inline copy of patch --
diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
index
caf80d66b3a744cc93899645aa5f9374983cd3db..3afd222ad3bdcfb922cc010dcc0b138db29caf7f
100644
--- a/gcc/config/aarch64/aarch64.cc
+++ b/gcc/config/aarch64/aarch64.cc
@@ -16388,12 +16388,22 @@ aarch64_override_options (void)
if (cpu && arch)
{
/* If both -mcpu and -march are specified, warn if they are not
- architecturally compatible and prefer the -march ISA flags. */
- if (arch->arch != cpu->arch)
- {
- warning (0, "switch %<-mcpu=%s%> conflicts with %<-march=%s%> switch",
+ feature compatible. feature compatible means that the inclusion of the
+ cpu features would end up disabling an achitecture feature. In
"CPU" and "architecture"
+ otherwords the cpu features need to be a strict superset of the arch
"other words" and "CPU".
+ features and if so prefer the -march ISA flags. */
+ auto full_arch_flags = arch->flags | arch_isa;
+ auto full_cpu_flags = cpu->flags | cpu_isa;
+ if (~full_cpu_flags & full_arch_flags)
+ {
+ std::string ext_diff
+ = aarch64_get_extension_string_for_isa_flags (full_arch_flags,
+ full_cpu_flags);
+ warning (0, "switch %<-mcpu=%s%> conflicts with %<-march=%s%> switch "
+ "and resulted in options %s being added",
Please check the convention here: should %s be surrounded in %<..%>? It
is part of what the user effectively specified on the command line.
aarch64_cpu_string,
- aarch64_arch_string);
+ aarch64_arch_string,
+ ext_diff.c_str ());
}
selected_arch = arch->arch;
Otherwise OK.
R.
