[TSan, SanitizerBinaryMetadata] Improve instrument for derived pointers via phis/selects

ThreadSanitizer.cpp and SanitizerBinaryMetadata.cpp previously used
`getUnderlyingObject` to check if pointers originate from stack objects.
However,
`getUnderlyingObject()` by default only looks through linear chains, not
selects/phis. In particular, this means that we miss cases involving
involving pointer induction variables.

This patch replaces `getUnderlyingObject` with `findAllocaForValue`
which:
1. Properly tracks through PHINodes and select operations
2. Directly identifies if a pointer comes from a `AllocaInst`

Performance impact:
- Compilation: Moderate cost increase due to wider value tracing, but...
- Runtime: Significant wins for code with pointer induction variables
derived from stack allocas, especially for loop-heavy code, as
instrumentation can now be safely omitted.

Author: Camsyn

llvm/lib/Transforms/Instrumentation/SanitizerBinaryMetadata.cpp

@@ -393,8 +393,8 @@ bool maybeSharedMutable(const Value *Addr) {
   if (!Addr)
     return true;
 
-  if (isa<AllocaInst>(getUnderlyingObject(Addr)) &&
-      !PointerMayBeCaptured(Addr, /*ReturnCaptures=*/true))
+  const AllocaInst *AI = findAllocaForValue(Addr);
+  if (AI && !PointerMayBeCaptured(Addr, /*ReturnCaptures=*/true))
     return false; // Object is on stack but does not escape.
 
   Addr = Addr->stripInBoundsOffsets();

llvm/lib/Transforms/Instrumentation/ThreadSanitizer.cpp

@@ -448,8 +448,8 @@ void ThreadSanitizer::chooseInstructionsToInstrument(
       }
     }
 
-    if (isa<AllocaInst>(getUnderlyingObject(Addr)) &&
-        !PointerMayBeCaptured(Addr, /*ReturnCaptures=*/true)) {
+    const AllocaInst *AI = findAllocaForValue(Addr);
+    if (AI && !PointerMayBeCaptured(Addr, /*ReturnCaptures=*/true)) {
       // The variable is addressable but not captured, so it cannot be
       // referenced from a different thread and participate in a data race
       // (see llvm/Analysis/CaptureTracking.h for details).

llvm/test/Instrumentation/ThreadSanitizer/capture.ll

@@ -88,4 +88,35 @@ entry:
 ; CHECK: __tsan_write
 ; CHECK: ret void
 
+define void @notcaptured3(i1 %cond) nounwind uwtable sanitize_thread {
+entry:
+  %stkobj = alloca [2 x i32], align 8
+  %derived = getelementptr inbounds i32, ptr %stkobj, i64 1
+  %ptr = select i1 %cond, ptr %derived, ptr %stkobj
+  store i32 42, ptr %ptr, align 4
+  ret void
+}
+; CHECK-LABEL: define void @notcaptured3
+; CHECK-NOT: call void @__tsan_write4(ptr %ptr)
+; CHECK: ret void
 
+define void @notcaptured4() nounwind uwtable sanitize_thread {
+entry:
+  %stkobj = alloca [10 x i8], align 1
+  br label %loop
+
+exit:
+  ret void
+
+loop:
+  %count = phi i32 [ 0, %entry ], [ %addone, %loop ]
+  %derived = phi ptr [ %stkobj, %entry ], [ %ptraddone, %loop ]
+  store i32 %count, ptr %derived, align 4
+  %ptraddone = getelementptr inbounds i32, ptr %derived, i64 1
+  %addone = add nuw nsw i32 %count, 1
+  %eq10 = icmp eq i32 %addone, 10
+  br i1 %eq10, label %exit, label %loop
+}
+; CHECK-LABEL: define void @notcaptured4
+; CHECK: ret void
+; CHECK-NOT: call void @__tsan_write4(ptr %derived)