Add comment in Memory.inc explaining r175646.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175650 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Support/Unix/Memory.inc b/lib/Support/Unix/Memory.inc
index a66e3c7aeee634623be343c20cb73f8662dd924a..e00394ec6aa34a9dffe5ded4e4ed6c9c685a6dea 100644 (file)
--- a/lib/Support/Unix/Memory.inc
+++ b/lib/Support/Unix/Memory.inc
@@ -52,6 +52,13 @@ int getPosixProtectionFlags(unsigned Flags) {
     return PROT_READ | PROT_WRITE | PROT_EXEC;
   case llvm::sys::Memory::MF_EXEC:
 #if defined(__FreeBSD__)
+    // On PowerPC, having an executable page that has no read permission
+    // can have unintended consequences.  The function InvalidateInstruction-
+    // Cache uses instructions dcbf and icbi, both of which are treated by
+    // the processor as loads.  If the page has no read permissions,
+    // executing these instructions will result in a segmentation fault.
+    // Somehow, this problem is not present on Linux, but it does happen
+    // on FreeBSD.
     return PROT_READ | PROT_EXEC;
 #else
     return PROT_EXEC;