[ARM] Improve if-conversion for M-class CPUs without branch predictors
The current heuristic in isProfitableToIfCvt assumes we have a branch predictor,
and so gives the wrong answer in some cases when we don't. This patch adds a
subtarget feature to indicate that a subtarget has no branch predictor, and
changes the heuristic in isProfitableToiIfCvt when it's present. This gives a
slight overall improvement in a set of embedded benchmarks on Cortex-M4 and
Cortex-M33.
Differential Revision: https://reviews.llvm.org/D34398
llvm-svn: 306547
diff --git a/llvm/lib/Target/ARM/ARMSubtarget.h b/llvm/lib/Target/ARM/ARMSubtarget.h
index d890d0f..e15b175 100644
--- a/llvm/lib/Target/ARM/ARMSubtarget.h
+++ b/llvm/lib/Target/ARM/ARMSubtarget.h
@@ -246,6 +246,11 @@
/// avoid issue "normal" call instructions to callees which do not return.
bool HasRetAddrStack = false;
+ /// HasBranchPredictor - True if the subtarget has a branch predictor. Having
+ /// a branch predictor or not changes the expected cost of taking a branch
+ /// which affects the choice of whether to use predicated instructions.
+ bool HasBranchPredictor = true;
+
/// HasMPExtension - True if the subtarget supports Multiprocessing
/// extension (ARMv7 only).
bool HasMPExtension = false;
@@ -554,6 +559,7 @@
bool cheapPredicableCPSRDef() const { return CheapPredicableCPSRDef; }
bool avoidMOVsShifterOperand() const { return AvoidMOVsShifterOperand; }
bool hasRetAddrStack() const { return HasRetAddrStack; }
+ bool hasBranchPredictor() const { return HasBranchPredictor; }
bool hasMPExtension() const { return HasMPExtension; }
bool hasDSP() const { return HasDSP; }
bool useNaClTrap() const { return UseNaClTrap; }