[compiler-rt] [test] Fix page address logic in clear_cache_test

[mgorny]

Summary

Fix the logic used to calculate page boundaries in clear_cache_test to
use correct masks -- e.g. -4096 rather than -4095. The latter gives
incorrect result since:

-4095 -> 0xfffff001
-4096 -> 0xfffff000 (== ~4095)

The issue went unnoticed so far because the array alignment caused
the last bit not to be set. However, on 32-bit x86 no such alignment is
enforced and the wrong page address caused the test to fail.

Furthermore, obtain the page size from the system instead of hardcoding
4096.

[compnerd]

I wonder if we should take this a bit further. Use something like a PAGE_SIZE constant to avoid this confusion entirely. Furthremore, different targets could use different page sizes. IIRC, SPARC has a 8K page size by default.

[mgorny]

Updated to use page size logic on POSIX and Windows systems. I haven't tested the latter, though.

[compnerd]

test/builtins/Unit/clear_cache_test.c

60

Shouldnt this read:

char *end = (char *)((uintptr_t)(&execution_buffer[128 + get_page_size()] & (-get_page_size())));

[mgorny]

Fixed the missing change. Also created a local variable to avoid calling the sysconf three times.

[compnerd]

Would be nice to clang-format those lines, but its no worse than before.

test/builtins/Unit/clear_cache_test.c

 #endif
 }
 
 unsigned char execution_buffer[128];
 
 int main()
 {
     // make executable the page containing execution_buffer 
-    char* start = (char*)((uintptr_t)execution_buffer & (-4095));
-    char* end = (char*)((uintptr_t)(&execution_buffer[128+4096]) & (-4095));
+    char* start = (char*)((uintptr_t)execution_buffer & (~4095));
+    char* end = (char*)((uintptr_t)(&execution_buffer[128+4096]) & (~4095));
 #if defined(_WIN32)
     DWORD dummy_oldProt;
     MEMORY_BASIC_INFORMATION b;
     if (!VirtualQuery(start, &b, sizeof(b)))
         return 1;
     if (!VirtualProtect(b.BaseAddress, b.RegionSize, PAGE_EXECUTE_READWRITE, &b.Protect))
 #else
     if (mprotect(start, end-start, PROT_READ|PROT_WRITE|PROT_EXEC) != 0)