"float2int": Add a new pass to demote from float to int where possible.

It is possible to have code that converts from integer to float, performs operations then converts back, and the result is provably the same as if integers were used.

This can come from different sources, but the most obvious is a helper function that uses floats but the arguments given at an inlined callsites are integers.

This pass considers all integers requiring a bitwidth less than or equal to the bitwidth of the mantissa of a floating point type (23 for floats, 52 for doubles) as exactly representable in floating point.

To reduce the risk of harming efficient code, the pass only attempts to perform complete removal of inttofp/fptoint operations, not just move them around.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233062 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/IPO/PassManagerBuilder.cpp

@@ -59,6 +59,10 @@ static cl::opt<bool>
 RunLoopRerolling("reroll-loops", cl::Hidden,
                  cl::desc("Run the loop rerolling pass"));
 
+static cl::opt<bool>
+RunFloat2Int("float-to-int", cl::Hidden, cl::init(true),
+             cl::desc("Run the float2int (float demotion) pass"));
+
 static cl::opt<bool> RunLoadCombine("combine-loads", cl::init(false),
                                     cl::Hidden,
                                     cl::desc("Run the load combining pass"));
@@ -307,6 +311,9 @@ void PassManagerBuilder::populateModulePassManager(
   // we must insert a no-op module pass to reset the pass manager.
   MPM.add(createBarrierNoopPass());
 
+  if (RunFloat2Int)
+    MPM.add(createFloat2IntPass());
+
   // Re-rotate loops in all our loop nests. These may have fallout out of
   // rotated form due to GVN or other transformations, and the vectorizer relies
   // on the rotated form.