x86/idt: Minor improvements to _update_gate_addr_lower()

After some experimentation, using .a/.b makes far better logic than using the
named fields, as both Clang and GCC spill idte to the stack when named fields
are used.

GCC seems to do something very daft for the addr1 field.  It takes addr,
shifts it by 32, then ANDs with 0xffff0000000000000UL, which requires
manifesting a MOVABS.

Clang follows the C, whereby it ANDs with $imm32, then shifts, avoiding the
MOVABS entirely.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/include/asm/idt.h

@@ -92,15 +92,16 @@ static inline void _set_gate_lower(idt_entry_t *gate, unsigned long type,
  * Update the lower half handler of an IDT entry, without changing any other
  * configuration.
  */
-static inline void _update_gate_addr_lower(idt_entry_t *gate, void *addr)
+static inline void _update_gate_addr_lower(idt_entry_t *gate, void *_addr)
 {
+    unsigned long addr = (unsigned long)_addr;
+    unsigned int addr1 = addr & 0xffff0000U; /* GCC: force better codegen. */
     idt_entry_t idte;
-    idte.a = gate->a;
 
-    idte.b = ((unsigned long)(addr) >> 32);
-    idte.a &= 0x0000FFFFFFFF0000ULL;
-    idte.a |= (((unsigned long)(addr) & 0xFFFF0000UL) << 32) |
-        ((unsigned long)(addr) & 0xFFFFUL);
+    idte.b = addr >> 32;
+    idte.a = gate->a & 0x0000ffffffff0000UL;
+    idte.a |= (unsigned long)addr1 << 32;
+    idte.a |= addr & 0xffff;
 
     _write_gate_lower(gate, &idte);
 }