[memprof] Only insert dynamic shadow load when needed

[tejohnson]

Summary

We don't need to insert a load of the dynamic shadow address unless there
are interesting memory accesses to profile.

Split out of D124703.

[snehasish]

lgtm

llvm/test/Instrumentation/HeapProfiler/no-instrumentation.ll

15

Can we also add a check for the log line "MEMPROF done instrumenting:"? Otherwise this test won't fail if memprof instrumentation doesn't run.

[tejohnson]

Address comment

llvm/lib/Transforms/Instrumentation/MemProfiler.cpp

bool MemProfiler::instrumentFunction(Function &F) {
   // instrumented.
   if (maybeInsertMemProfInitAtFunctionEntry(F))
     FunctionModified = true;
 
   LLVM_DEBUG(dbgs() << "MEMPROF instrumenting:\n" << F << "\n");
 
   initializeCallbacks(*F.getParent());
 
-  FunctionModified |= insertDynamicShadowAtFunctionEntry(F);
-
   SmallVector<Instruction *, 16> ToInstrument;
 
   // Fill the set of memory operations to instrument.
   for (auto &BB : F) {
     for (auto &Inst : BB) {
       if (isInterestingMemoryAccess(&Inst) || isa<MemIntrinsic>(Inst))
         ToInstrument.push_back(&Inst);
     }
   }
 
+  if (ToInstrument.empty()) {
+    LLVM_DEBUG(dbgs() << "MEMPROF done instrumenting: " << FunctionModified
+                      << " " << F << "\n");
+
+    return FunctionModified;
+  }
+
+  FunctionModified |= insertDynamicShadowAtFunctionEntry(F);
+
   int NumInstrumented = 0;
   for (auto *Inst : ToInstrument) {
     if (ClDebugMin < 0 || ClDebugMax < 0 ||
         (NumInstrumented >= ClDebugMin && NumInstrumented <= ClDebugMax)) {
       Optional<InterestingMemoryAccess> Access =
           isInterestingMemoryAccess(Inst);
       if (Access)
         instrumentMop(Inst, F.getParent()->getDataLayout(), *Access);

llvm/test/Instrumentation/HeapProfiler/no-instrumentation.ll

+;; Test that we don't add any instrumentation code to functions without
+;; interesting memory accesses.
+;
+; RUN: opt < %s -passes='function(memprof),module(memprof-module)' -S -debug 2>&1 | FileCheck %s
+
+;; Require asserts for -debug
+; REQUIRES: asserts
+
+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"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @_Z3foov() {
+entry:
+  ret void
+}
+
+;; Confirm we ran memprof and decided not to instrument
+; CHECK: MEMPROF done instrumenting: 0 define void @_Z3foov
+
+;; We should not add any instrumentation related code
+; CHECK: define void @_Z3foov
+; CHECK-NEXT: entry:
+; CHECK-NEXT:  ret void