[FastISel] Disable local value sinking by default

This is causing compilation timeouts on code with long sequences of
local values and calls (i.e. foo(1); foo(2); foo(3); ...).  It turns out
that code coverage instrumentation is a great way to create sequences
like this, which how our users ran into the issue in practice.

Intel has a tool that detects these kinds of non-linear compile time
issues, and Andy Kaylor reported it as PR37010.

The current sinking code scans the whole basic block once per local
value sink, which happens before emitting each call. In theory, local
values should only be introduced to be used by instructions between the
current flush point and the last flush point, so we should only need to
scan those instructions.

llvm-svn: 329822

1 files changed, 8 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
index 601691a795a..a88359ff27a 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -113,6 +113,13 @@ using namespace llvm;
 
 #define DEBUG_TYPE "isel"
 
+// FIXME: Remove this when compile time issues are addressed. Do this by only
+// numbering instructions between local value map flush points instead of the
+// entire BB.
+static cl::opt<bool> SinkLocalValues("fast-isel-sink-local-values",
+                                     cl::init(false), cl::Hidden,
+                                     cl::desc("Sink local values in FastISel"));
+
 STATISTIC(NumFastIselSuccessIndependent, "Number of insts selected by "
                                          "target-independent selector");
 STATISTIC(NumFastIselSuccessTarget, "Number of insts selected by "
@@ -180,7 +187,7 @@ void FastISel::flushLocalValueMap() {
   // Try to sink local values down to their first use so that we can give them a
   // better debug location. This has the side effect of shrinking local value
   // live ranges, which helps out fast regalloc.
-  if (LastLocalValue != EmitStartPt) {
+  if (SinkLocalValues && LastLocalValue != EmitStartPt) {
     // Sink local value materialization instructions between EmitStartPt and
     // LastLocalValue. Visit them bottom-up, starting from LastLocalValue, to
     // avoid inserting into the range that we're iterating over.