[clang] Remove old Linux kernel workaround for ensuring stack space (#81533)

PR #71709 broke the Linux PIE build with `undefined symbol: alloca`
errors. With the newly included `clang/Basic/Builtins.h` in that PR, it
surfaces an issue with a combination of two previous patches.

26670dc added `#undef alloca` so clang
builtins handling of alloca would work under MSVC (unsure if this is
still necessary).

194b6a3 added code that calls `alloca`
to workaround a Linux kernel < 4.1 bug. Given that Linux 4.1 was EOL in
2018, it should be ok to remove this workaround.

Author: aeubanks

Diff for: clang/tools/driver/cc1_main.cpp

@@ -78,64 +78,6 @@ static void LLVMErrorHandler(void *UserData, const char *Message,
 }
 
 #ifdef CLANG_HAVE_RLIMITS
-#if defined(__linux__) && defined(__PIE__)
-static size_t getCurrentStackAllocation() {
-  // If we can't compute the current stack usage, allow for 512K of command
-  // line arguments and environment.
-  size_t Usage = 512 * 1024;
-  if (FILE *StatFile = fopen("/proc/self/stat", "r")) {
-    // We assume that the stack extends from its current address to the end of
-    // the environment space. In reality, there is another string literal (the
-    // program name) after the environment, but this is close enough (we only
-    // need to be within 100K or so).
-    unsigned long StackPtr, EnvEnd;
-    // Disable silly GCC -Wformat warning that complains about length
-    // modifiers on ignored format specifiers. We want to retain these
-    // for documentation purposes even though they have no effect.
-#if defined(__GNUC__) && !defined(__clang__)
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wformat"
-#endif
-    if (fscanf(StatFile,
-               "%*d %*s %*c %*d %*d %*d %*d %*d %*u %*lu %*lu %*lu %*lu %*lu "
-               "%*lu %*ld %*ld %*ld %*ld %*ld %*ld %*llu %*lu %*ld %*lu %*lu "
-               "%*lu %*lu %lu %*lu %*lu %*lu %*lu %*lu %*llu %*lu %*lu %*d %*d "
-               "%*u %*u %*llu %*lu %*ld %*lu %*lu %*lu %*lu %*lu %*lu %lu %*d",
-               &StackPtr, &EnvEnd) == 2) {
-#if defined(__GNUC__) && !defined(__clang__)
-#pragma GCC diagnostic pop
-#endif
-      Usage = StackPtr < EnvEnd ? EnvEnd - StackPtr : StackPtr - EnvEnd;
-    }
-    fclose(StatFile);
-  }
-  return Usage;
-}
-
-#include <alloca.h>
-
-LLVM_ATTRIBUTE_NOINLINE
-static void ensureStackAddressSpace() {
-  // Linux kernels prior to 4.1 will sometimes locate the heap of a PIE binary
-  // relatively close to the stack (they are only guaranteed to be 128MiB
-  // apart). This results in crashes if we happen to heap-allocate more than
-  // 128MiB before we reach our stack high-water mark.
-  //
-  // To avoid these crashes, ensure that we have sufficient virtual memory
-  // pages allocated before we start running.
-  size_t Curr = getCurrentStackAllocation();
-  const int kTargetStack = DesiredStackSize - 256 * 1024;
-  if (Curr < kTargetStack) {
-    volatile char *volatile Alloc =
-        static_cast<volatile char *>(alloca(kTargetStack - Curr));
-    Alloc[0] = 0;
-    Alloc[kTargetStack - Curr - 1] = 0;
-  }
-}
-#else
-static void ensureStackAddressSpace() {}
-#endif
-
 /// Attempt to ensure that we have at least 8MiB of usable stack space.
 static void ensureSufficientStack() {
   struct rlimit rlim;
@@ -159,10 +101,6 @@ static void ensureSufficientStack() {
         rlim.rlim_cur != DesiredStackSize)
       return;
   }
-
-  // We should now have a stack of size at least DesiredStackSize. Ensure
-  // that we can actually use that much, if necessary.
-  ensureStackAddressSpace();
 }
 #else
 static void ensureSufficientStack() {}