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

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

Author: melver (Marco Elver)
Reviewed-in: https://reviews.llvm.org/D79983

llvm/lib/Transforms/Instrumentation/ThreadSanitizer.cpp

@@ -72,6 +72,10 @@ static cl::opt<bool>  ClDistinguishVolatile(
     "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 @@ void ThreadSanitizer::chooseInstructionsToInstrument(
       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;

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 @@ entry:
 }
 ; CHECK: define void @IncrementMe
 ; CHECK-NOT: __tsan_read
+; CHECK-UNOPT: __tsan_read
 ; CHECK: __tsan_write
 ; CHECK: ret void