[IfCvt][ARM] Optimise diamond if-conversion for code size

Currently, the heuristics the if-conversion pass uses for diamond if-conversion
are based on execution time, with no consideration for code size. This adds a
new set of heuristics to be used when optimising for code size.

This is mostly target-independent, because the if-conversion pass can
see the code size of the instructions which it is removing. For thumb,
there are a few passes (insertion of IT instructions, selection of
narrow branches, and selection of CBZ instructions) which are run after
if conversion and affect these heuristics, so I've added target hooks to
better predict the code-size effect of a proposed if-conversion.

Differential revision: https://reviews.llvm.org/D67350

llvm-svn: 374301

1 files changed, 32 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp b/llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp
index de53cb3b59d..684cd1def97 100644
--- a/llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp
+++ b/llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp
@@ -2079,6 +2079,38 @@ isProfitableToIfCvt(MachineBasicBlock &TBB,
   return PredCost <= UnpredCost;
 }
 
+unsigned
+ARMBaseInstrInfo::extraSizeToPredicateInstructions(const MachineFunction &MF,
+                                                   unsigned NumInsts) const {
+  // Thumb2 needs a 2-byte IT instruction to predicate up to 4 instructions.
+  // ARM has a condition code field in every predicable instruction, using it
+  // doesn't change code size.
+  return Subtarget.isThumb2() ? divideCeil(NumInsts, 4) * 2 : 0;
+}
+
+unsigned
+ARMBaseInstrInfo::predictBranchSizeForIfCvt(MachineInstr &MI) const {
+  // If this branch is likely to be folded into the comparison to form a
+  // CB(N)Z, then removing it won't reduce code size at all, because that will
+  // just replace the CB(N)Z with a CMP.
+  if (MI.getOpcode() == ARM::t2Bcc &&
+      findCMPToFoldIntoCBZ(&MI, &getRegisterInfo()))
+    return 0;
+
+  unsigned Size = getInstSizeInBytes(MI);
+
+  // For Thumb2, all branches are 32-bit instructions during the if conversion
+  // pass, but may be replaced with 16-bit instructions during size reduction.
+  // Since the branches considered by if conversion tend to be forward branches
+  // over small basic blocks, they are very likely to be in range for the
+  // narrow instructions, so we assume the final code size will be half what it
+  // currently is.
+  if (Subtarget.isThumb2())
+    Size /= 2;
+
+  return Size;
+}
+
 bool
 ARMBaseInstrInfo::isProfitableToUnpredicate(MachineBasicBlock &TMBB,
                                             MachineBasicBlock &FMBB) const {