minmax: avoid overly complex min()/max() macro arguments in xen

We have some very fancy min/max macros that have tons of sanity checking
to warn about mixed signedness etc.

This is all things that a sane compiler should warn about, but there are
no sane compiler interfaces for this, and '-Wsign-compare' is broken [1]
and not useful.

So then we compensate (some would say over-compensate) by doing the
checks manually with some truly horrid macro games.

And no, we can't just use __builtin_types_compatible_p(), because the
whole question of "does it make sense to compare these two values" is a
lot more complicated than that.

For example, it makes a ton of sense to compare unsigned values with
simple constants like "5", even if that is indeed a signed type.  So we
have these very strange macros to try to make sensible type checking
decisions on the arguments to 'min()' and 'max()'.

But that can cause enormous code expansion if the min()/max() macros are
used with complicated expressions, and particularly if you nest these
things so that you get the first big expansion then expanded again.

The xen setup.c file ended up ballooning to over 50MB of preprocessed
noise that takes 15s to compile (obviously depending on the build host),
largely due to one single line.

So let's split that one single line to just be simpler.  I think it ends
up being more legible to humans too at the same time.  Now that single
file compiles in under a second.

Reported-and-reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Link: https://lore.kernel.org/all/c83c17bb-be75-4c67-979d-54eee38774c6@lucifer.local/
Link: https://staticthinking.wordpress.com/2023/07/25/wsign-compare-is-garbage/ [1]
Cc: David Laight <David.Laight@aculab.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 2 deletions

diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index a0c3e77e3d5b..806ddb2391d9 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -690,6 +690,7 @@ char * __init xen_memory_setup(void)
 	struct xen_memory_map memmap;
 	unsigned long max_pages;
 	unsigned long extra_pages = 0;
+	unsigned long maxmem_pages;
 	int i;
 	int op;
 
@@ -761,8 +762,8 @@ char * __init xen_memory_setup(void)
 	 * Make sure we have no memory above max_pages, as this area
 	 * isn't handled by the p2m management.
 	 */
-	extra_pages = min3(EXTRA_MEM_RATIO * min(max_pfn, PFN_DOWN(MAXMEM)),
-			   extra_pages, max_pages - max_pfn);
+	maxmem_pages = EXTRA_MEM_RATIO * min(max_pfn, PFN_DOWN(MAXMEM));
+	extra_pages = min3(maxmem_pages, extra_pages, max_pages - max_pfn);
 	i = 0;
 	addr = xen_e820_table.entries[0].addr;
 	size = xen_e820_table.entries[0].size;