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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154263 91177308-0d34-0410-b5e6-96231b3b80d8

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:

test/Transforms/LoopRotate/dbgvalue.ll

@@ -1,11 +1,13 @@
 ; RUN: opt -S -loop-rotate  %s  | FileCheck %s
 
+declare void @llvm.dbg.declare(metadata, metadata) nounwind readnone
+declare void @llvm.dbg.value(metadata, i64, metadata) nounwind readnone
+
+define i32 @tak(i32 %x, i32 %y, i32 %z) nounwind ssp {
+; CHECK: define i32 @tak
 ; CHECK: entry
 ; CHECK-NEXT: call void @llvm.dbg.value(metadata !{i32 %x}
 
-declare void @llvm.dbg.declare(metadata, metadata) nounwind readnone
-
-define i32 @tak(i32 %x, i32 %y, i32 %z) nounwind ssp {
 entry:
   br label %tailrecurse
 
@@ -35,7 +37,45 @@ return:                                           ; preds = %if.end
   ret i32 %z.tr, !dbg !17
 }
 
-declare void @llvm.dbg.value(metadata, i64, metadata) nounwind readnone
+@channelColumns = external global i64
+@horzPlane = external global i8*, align 8
+
+define void @FindFreeHorzSeg(i64 %startCol, i64 %row, i64* %rowStart) {
+; Ensure that the loop increment basic block is rotated into the tail of the
+; body, even though it contains a debug intrinsic call.
+; CHECK: define void @FindFreeHorzSeg
+; CHECK: %dec = add
+; CHECK-NEXT: tail call void @llvm.dbg.value
+; CHECK-NEXT: br i1 %tobool, label %for.cond, label %for.end
+
+entry:
+  br label %for.cond
+
+for.cond:
+  %i.0 = phi i64 [ %startCol, %entry ], [ %dec, %for.inc ]
+  %cmp = icmp eq i64 %i.0, 0
+  br i1 %cmp, label %for.end, label %for.body
+
+for.body:
+  %0 = load i64* @channelColumns, align 8
+  %mul = mul i64 %0, %row
+  %add = add i64 %mul, %i.0
+  %1 = load i8** @horzPlane, align 8
+  %arrayidx = getelementptr inbounds i8* %1, i64 %add
+  %2 = load i8* %arrayidx, align 1
+  %tobool = icmp eq i8 %2, 0
+  br i1 %tobool, label %for.inc, label %for.end
+
+for.inc:
+  %dec = add i64 %i.0, -1
+  tail call void @llvm.dbg.value(metadata !{i64 %dec}, i64 0, metadata undef)
+  br label %for.cond
+
+for.end:
+  %add1 = add i64 %i.0, 1
+  store i64 %add1, i64* %rowStart, align 8
+  ret void
+}
 
 !llvm.dbg.sp = !{!0}