[PATCH] rombios: Work around GCC issue 99578

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

GCC 12 objects to pointers derived from a constant:

  util.c: In function 'find_rsdp':
  util.c:429:16: error: array subscript 0 is outside array bounds of 
'uint16_t[0]' {aka 'short unsigned int[]'} [-Werror=array-bounds]
    429 |     ebda_seg = *(uint16_t *)ADDR_FROM_SEG_OFF(0x40, 0xe);
  cc1: all warnings being treated as errors

This is a GCC bug, but work around it rather than turning array-bounds
checking off generally.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>

This only manifests in release builds, presumably a side effect of neediung
some constant-folding to notice the ADDR_FROM_SEG_OFF() expression.

We don't see this in CI because debug isn't configured correctly for the
tools/ part of the build
---
 tools/firmware/rombios/32bit/util.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tools/firmware/rombios/32bit/util.c 
b/tools/firmware/rombios/32bit/util.c
index 6c1c4805144b..a47e000a2629 100644
--- a/tools/firmware/rombios/32bit/util.c
+++ b/tools/firmware/rombios/32bit/util.c
@@ -424,10 +424,10 @@ static struct acpi_20_rsdp *__find_rsdp(const void 
*start, unsigned int len)
 struct acpi_20_rsdp *find_rsdp(void)
 {
     struct acpi_20_rsdp *rsdp;
-    uint16_t ebda_seg;
+    uint16_t *volatile /* GCC issue 99578 */ ebda_seg =
+        ADDR_FROM_SEG_OFF(0x40, 0xe);
 
-    ebda_seg = *(uint16_t *)ADDR_FROM_SEG_OFF(0x40, 0xe);
-    rsdp = __find_rsdp((void *)(ebda_seg << 16), 1024);
+    rsdp = __find_rsdp((void *)(*ebda_seg << 16), 1024);
     if (!rsdp)
         rsdp = __find_rsdp((void *)0xE0000, 0x20000);
 

base-commit: d0eabe3eaf0db5b78843095a2918d50961e99e96
-- 
2.30.2