[clang-tools-extra] [compiler-rt] [lldb] [llvm] [Memprof] Adds the option to collect AccessCountHistograms for memprof. (PR #94264)

[Teresa Johnson via cfe-commits]

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@@ -216,6 +228,35 @@ u64 GetShadowCount(uptr p, u32 size) {
   return count;
 }
 
+// Accumulates the access count from the shadow for the given pointer and size.
+// See memprof_mapping.h for an overview on histogram counters.
+u64 GetShadowCountHistogram(uptr p, u32 size) {
+  u8 *shadow = (u8 *)HISTOGRAM_MEM_TO_SHADOW(p);
+  u8 *shadow_end = (u8 *)HISTOGRAM_MEM_TO_SHADOW(p + size);
+  u64 count = 0;
+  for (; shadow <= shadow_end; shadow++)
+    count += *shadow;
+  return count;
+}
+
+// If we use the normal approach from clearCountersWithoutHistogram, the
+// histogram will clear too much data and may overwrite shadow counters that are
+// in use. Likely because of rounding up the shadow_end pointer.
+// See memprof_mapping.h for an overview on histogram counters.
+void clearCountersHistogram(uptr addr, uptr size) {
+  u8 *shadow_8 = (u8 *)HISTOGRAM_MEM_TO_SHADOW(addr);
+  u8 *shadow_end_8 = (u8 *)HISTOGRAM_MEM_TO_SHADOW(addr + size);
+  for (; shadow_8 < shadow_end_8; shadow_8++) {
+    *shadow_8 = 0;
+  }
+}
+
+void clearCountersWithoutHistogram(uptr addr, uptr size) {
+  uptr shadow_beg = MEM_TO_SHADOW(addr);
+  uptr shadow_end = MEM_TO_SHADOW(addr + size - SHADOW_GRANULARITY) + 1;
+  REAL(memset)((void *)shadow_beg, 0, shadow_end - shadow_beg);
+}
+
 // Clears the shadow counters (when memory is allocated).
 void ClearShadow(uptr addr, uptr size) {
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teresajohnson wrote:

Looking at this function more, there are a lot of uses of the MEM_TO_SHADOW computed values, which is not the right mapping function to use with the smaller granularity of the histogram case. I think you probably need to version the calls to MEM_TO_SHADOW here, then all of the rest of the code can work as is? I.e. you wouldn't need the 2 versions of `clearCounters*`

https://github.com/llvm/llvm-project/pull/94264