Fix ValueTracking to conclude that debug intrinsics are safe to

speculate. Without this, loop rotate (among many other places) would
suddenly stop working in the presence of debug info. I found this
looking at loop rotate, and have augmented its tests with a reduction
out of a very hot loop in yacr2 where failing to do this rotation costs
sometimes more than 10% in runtime performance, perturbing numerous
downstream optimizations.

This should have no impact on performance without debug info, but the
change in performance when debug info is enabled can be extreme. As
a consequence (and this how I got to this yak) any profiling of
performance problems should be treated with deep suspicion -- they may
have been wildly innacurate of debug info was enabled for profiling. =/
Just a heads up.

llvm-svn: 154263

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index c6b53a927db..a430f6281ef 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -1854,6 +1854,14 @@ bool llvm::isSafeToSpeculativelyExecute(const Value *V,
   case Instruction::Call: {
    if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst)) {
      switch (II->getIntrinsicID()) {
+       // These synthetic intrinsics have no side-effects, and just mark
+       // information about their operands.
+       // FIXME: There are other no-op synthetic instructions that potentially
+       // should be considered at least *safe* to speculate...
+       case Intrinsic::dbg_declare:
+       case Intrinsic::dbg_value:
+         return true;
+
        case Intrinsic::bswap:
        case Intrinsic::ctlz:
        case Intrinsic::ctpop: