[PATCH 6/6]AArch64: only emit mismatch error when features would be disabled.

[Tamar Christina <tamar.christina@arm.com>]

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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?

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
+	 otherwords the cpu features need to be a strict superset of the arch
+	 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",
 		       aarch64_cpu_string,
-		       aarch64_arch_string);
+		       aarch64_arch_string,
+		       ext_diff.c_str ());
 	}
 
       selected_arch = arch->arch;




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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
+	 otherwords the cpu features need to be a strict superset of the arch
+	 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",
 		       aarch64_cpu_string,
-		       aarch64_arch_string);
+		       aarch64_arch_string,
+		       ext_diff.c_str ());
 	}
 
       selected_arch = arch->arch;