[PATCH] D79983: [TSAN] Add option to allow instrumenting reads of reads-before-writes

[Marco Elver via Phabricator via llvm-commits]

melver created this revision.
melver added a reviewer: dvyukov.
Herald added subscribers: llvm-commits, jfb, hiraditya.
Herald added a project: LLVM.
melver added a reviewer: glider.

Add -tsan-instrument-read-before-write which allows instrumenting reads
of reads-before-writes.

This is required for KCSAN [1], where under certain configurations plain
writes behave differently (e.g. aligned writes up to word size may be
treated as atomic). In order to avoid missing potential data races due
to plain RMW operations ("x++" etc.), we will require instrumenting
reads of reads-before-writes.

[1] https://github.com/google/ktsan/wiki/KCSAN


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D79983

Files:
  llvm/lib/Transforms/Instrumentation/ThreadSanitizer.cpp
  llvm/test/Instrumentation/ThreadSanitizer/read_before_write.ll


Index: llvm/test/Instrumentation/ThreadSanitizer/read_before_write.ll
===================================================================
--- llvm/test/Instrumentation/ThreadSanitizer/read_before_write.ll
+++ llvm/test/Instrumentation/ThreadSanitizer/read_before_write.ll
@@ -1,4 +1,5 @@
 ; RUN: opt < %s -tsan -S | FileCheck %s
+; RUN: opt < %s -tsan -tsan-instrument-read-before-write -S | FileCheck %s --check-prefixes=CHECK,CHECK-UNOPT
 
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
 
@@ -11,6 +12,7 @@
 }
 ; CHECK: define void @IncrementMe
 ; CHECK-NOT: __tsan_read
+; CHECK-UNOPT: __tsan_read
 ; CHECK: __tsan_write
 ; CHECK: ret void
 
Index: llvm/lib/Transforms/Instrumentation/ThreadSanitizer.cpp
===================================================================
--- llvm/lib/Transforms/Instrumentation/ThreadSanitizer.cpp
+++ llvm/lib/Transforms/Instrumentation/ThreadSanitizer.cpp
@@ -72,6 +72,10 @@
     "tsan-distinguish-volatile", cl::init(false),
     cl::desc("Emit special instrumentation for accesses to volatiles"),
     cl::Hidden);
+static cl::opt<bool>  ClInstrumentReadBeforeWrite(
+    "tsan-instrument-read-before-write", cl::init(false),
+    cl::desc("Do not eliminate read instrumentation for read-before-writes"),
+    cl::Hidden);
 
 STATISTIC(NumInstrumentedReads, "Number of instrumented reads");
 STATISTIC(NumInstrumentedWrites, "Number of instrumented writes");
@@ -413,7 +417,7 @@
       Value *Addr = Load->getPointerOperand();
       if (!shouldInstrumentReadWriteFromAddress(I->getModule(), Addr))
         continue;
-      if (WriteTargets.count(Addr)) {
+      if (!ClInstrumentReadBeforeWrite && WriteTargets.count(Addr)) {
         // We will write to this temp, so no reason to analyze the read.
         NumOmittedReadsBeforeWrite++;
         continue;


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