ARM: override cost function to re-enable ConstantHoisting (& fix it).

At some point, ARM stopped getting any benefit from ConstantHoisting because
the pass called a different variant of getIntImmCost. Reimplementing the
correct variant revealed some problems, however:

  + ConstantHoisting was modifying switch statements. This is simply invalid,
    the cases must remain integer constants no matter the notional cost.
  + ConstantHoisting was mangling alloca instructions in the entry block. These
    should be handled by FrameLowering, so constants actually have a cost of 0.
    Worse, the resulting bitcasts meant they became dynamic allocas.

rdar://25707382

llvm-svn: 266260

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp b/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp
index 84f7f5fff5b..d549228757f 100644
--- a/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp
+++ b/llvm/lib/Transforms/Scalar/ConstantHoisting.cpp
@@ -320,6 +320,18 @@ void ConstantHoisting::collectConstantCandidates(ConstCandMapType &ConstCandMap,
     if (isa<InlineAsm>(Call->getCalledValue()))
       return;
 
+  // Switch cases must remain constant, and if the value being tested is
+  // constant the entire thing should disappear.
+  if (isa<SwitchInst>(Inst))
+    return;
+
+  // Static allocas (constant size in the entry block) are handled by
+  // prologue/epilogue insertion so they're free anyway. We definitely don't
+  // want to make them non-constant.
+  auto AI = dyn_cast<AllocaInst>(Inst);
+  if (AI && AI->isStaticAlloca())
+    return;
+
   // Scan all operands.
   for (unsigned Idx = 0, E = Inst->getNumOperands(); Idx != E; ++Idx) {
     Value *Opnd = Inst->getOperand(Idx);