[PATCH v3 4/7] x86: control memset() and memcpy() inlining

[Jan Beulich <jbeulich@xxxxxxxx>]

Stop the compiler from inlining non-trivial memset() and memcpy() (for
memset() see e.g. map_vcpu_info() or kimage_load_segments() for
examples). This way we even keep the compiler from using REP STOSQ /
REP MOVSQ when we'd prefer REP STOSB / REP MOVSB (when ERMS is
available).

With gcc10 this yields a modest .text size reduction (release build) of
around 2k.

Unfortunately these options aren't understood by the clang versions I
have readily available for testing with; I'm unaware of equivalents.

Note also that using cc-option-add is not an option here, or at least I
couldn't make things work with it (in case the option was not supported
by the compiler): The embedded comma in the option looks to be getting
in the way.

Requested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v3: Re-base.
v2: New.
---
The boundary values are of course up for discussion - I wasn't really
certain whether to use 16 or 32; I'd be less certain about using yet
larger values.

Similarly whether to permit the compiler to emit REP STOSQ / REP MOVSQ
for known size, properly aligned blocks is up for discussion.

--- a/xen/arch/x86/arch.mk
+++ b/xen/arch/x86/arch.mk
@@ -65,6 +65,9 @@ endif
 $(call cc-option-add,CFLAGS_stack_boundary,CC,-mpreferred-stack-boundary=3)
 export CFLAGS_stack_boundary
 
+CFLAGS += $(call 
cc-option,$(CC),-mmemcpy-strategy=unrolled_loop:16:noalign$(comma)libcall:-1:noalign)
+CFLAGS += $(call 
cc-option,$(CC),-mmemset-strategy=unrolled_loop:16:noalign$(comma)libcall:-1:noalign)
+
 ifeq ($(CONFIG_UBSAN),y)
 # Don't enable alignment sanitisation.  x86 has efficient unaligned accesses,
 # and various things (ACPI tables, hypercall pages, stubs, etc) are wont-fix.