Fix fast stack-unwinding for Power

[wschmidt]

Summary

PowerPC differs from other architectures in that the callee-saved return address is stored in the caller's stack frame, not the callee's. This patch adjusts the logic to find the LR in the correct place for PowerPC.

Patch joint with Bill Seurer. This replaces review.llvm.org/D9259.

[eugenis]

Awesome.
Thank you for taking your time and digging to the bottom of this.
Might want to factor out loading of the caller pc into a helper function, but I don't have a strong opinion either way.
LGTM.

[samsonov]

Nice. Thanks for the investigation!

[wschmidt]

Committed as r243467.

lib/sanitizer_common/sanitizer_stacktrace.h

 #define SANITIZER_STACKTRACE_H
 
 #include "sanitizer_internal_defs.h"
 
 namespace __sanitizer {
 
 static const u32 kStackTraceMax = 256;
 
-#if SANITIZER_LINUX && (defined(__aarch64__) || defined(__powerpc__) || \
-                        defined(__powerpc64__) || defined(__sparc__) || \
+#if SANITIZER_LINUX && (defined(__aarch64__) || defined(__sparc__) || \
                         defined(__mips__))
 # define SANITIZER_CAN_FAST_UNWIND 0
 #elif SANITIZER_WINDOWS
 # define SANITIZER_CAN_FAST_UNWIND 0
 #else
 # define SANITIZER_CAN_FAST_UNWIND 1
 #endif
 

lib/sanitizer_common/sanitizer_stacktrace.cc

void BufferedStackTrace::FastUnwindStack(uptr pc, uptr bp, uptr stack_top,
   uhwptr *frame = GetCanonicFrame(bp, stack_top, stack_bottom);
   // Lowest possible address that makes sense as the next frame pointer.
   // Goes up as we walk the stack.
   uptr bottom = stack_bottom;
   // Avoid infinite loop when frame == frame[0] by using frame > prev_frame.
   while (IsValidFrame((uptr)frame, stack_top, bottom) &&
          IsAligned((uptr)frame, sizeof(*frame)) &&
          size < max_depth) {
+#ifdef __powerpc__
+    uhwptr *caller_frame = (uhwptr*)frame[0];
+    if (!IsValidFrame((uptr)caller_frame, stack_top, bottom) ||
+        !IsAligned((uptr)caller_frame, sizeof(uhwptr)))
+      break;
+    uhwptr pc1 = caller_frame[2];
+#else
     uhwptr pc1 = frame[1];
+#endif
     if (pc1 != pc) {
       trace_buffer[size++] = (uptr) pc1;
     }
     bottom = (uptr)frame;
     frame = GetCanonicFrame((uptr)frame[0], stack_top, bottom);
   }
 }