Scaling up values in ARMBaseInstrInfo::isProfitableToIfCvt() before t…

…hey are scaled by a probability to avoid precision issue.

In ARMBaseInstrInfo::isProfitableToIfCvt(), there is a simple cost model in which the number of cycles is scaled by a probability to estimate the cost. However, when the number of cycles is small (which is usually the case), there is a precision issue after the computation. To avoid this issue, this patch scales those cycles by 1024 (chosen to make the multiplication a litter faster) before they are scaled by the probability. Other variables are also scaled up for the final comparison.

Differential Revision: http://reviews.llvm.org/D12742

llvm-svn: 248018

llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp

@@ -1670,11 +1670,14 @@ isProfitableToIfCvt(MachineBasicBlock &MBB,
   }
 
   // Attempt to estimate the relative costs of predication versus branching.
-  unsigned UnpredCost = Probability.scale(NumCycles);
-  UnpredCost += 1; // The branch itself
-  UnpredCost += Subtarget.getMispredictionPenalty() / 10;
-
-  return (NumCycles + ExtraPredCycles) <= UnpredCost;
+  // Here we scale up each component of UnpredCost to avoid precision issue when
+  // scaling NumCycles by Probability.
+  const unsigned ScalingUpFactor = 1024;
+  unsigned UnpredCost = Probability.scale(NumCycles * ScalingUpFactor);
+  UnpredCost += ScalingUpFactor; // The branch itself
+  UnpredCost += Subtarget.getMispredictionPenalty() * ScalingUpFactor / 10;
+
+  return (NumCycles + ExtraPredCycles) * ScalingUpFactor <= UnpredCost;
 }
 
 bool ARMBaseInstrInfo::
@@ -1687,13 +1690,17 @@ isProfitableToIfCvt(MachineBasicBlock &TMBB,
     return false;
 
   // Attempt to estimate the relative costs of predication versus branching.
-  unsigned TUnpredCost = Probability.scale(TCycles);
-  unsigned FUnpredCost = Probability.getCompl().scale(FCycles);
+  // Here we scale up each component of UnpredCost to avoid precision issue when
+  // scaling TCycles/FCycles by Probability.
+  const unsigned ScalingUpFactor = 1024;
+  unsigned TUnpredCost = Probability.scale(TCycles * ScalingUpFactor);
+  unsigned FUnpredCost =
+      Probability.getCompl().scale(FCycles * ScalingUpFactor);
   unsigned UnpredCost = TUnpredCost + FUnpredCost;
-  UnpredCost += 1; // The branch itself
-  UnpredCost += Subtarget.getMispredictionPenalty() / 10;
+  UnpredCost += 1 * ScalingUpFactor; // The branch itself
+  UnpredCost += Subtarget.getMispredictionPenalty() * ScalingUpFactor / 10;
 
-  return (TCycles + FCycles + TExtra + FExtra) <= UnpredCost;
+  return (TCycles + FCycles + TExtra + FExtra) * ScalingUpFactor <= UnpredCost;
 }
 
 bool

llvm/test/CodeGen/ARM/2013-10-11-select-stalls.ll

@@ -1,8 +1,13 @@
 ; REQUIRES: asserts
-; RUN: llc < %s -mtriple=thumbv7-apple-ios -stats 2>&1 | not grep "Number of pipeline stalls"
+; RUN: llc < %s -mtriple=thumbv7-apple-ios -disable-ifcvt-diamond -stats 2>&1 | FileCheck %s
 ; Evaluate the two vld1.8 instructions in separate MBB's,
 ; instead of stalling on one and conditionally overwriting its result.
+;
+; Update: After if-conversion the two vld1.8 instructions are in the same MBB
+; again. So we disable this if-conversion to eliminate its influence to this
+; test.
 
+; CHECK-NOT: Number of pipeline stalls
 define <16 x i8> @multiselect(i32 %avail, i8* %foo, i8* %bar) {
 entry:
   %vld1 = call <16 x i8> @llvm.arm.neon.vld1.v16i8(i8* %foo, i32 1)

llvm/test/CodeGen/ARM/ifcvt4.ll

@@ -1,10 +1,8 @@
 ; RUN: llc -mtriple=arm-eabi %s -o - | FileCheck %s
 
-; Do not if-convert when branches go to the different loops.
 ; CHECK-LABEL: t:
-; CHECK-NOT: subgt
-; CHECK-NOT: suble
-; Don't use
+; CHECK: subgt
+; CHECK: suble
 define i32 @t(i32 %a, i32 %b) {
 entry:
 	%tmp1434 = icmp eq i32 %a, %b		; <i1> [#uses=1]

llvm/test/CodeGen/ARM/sjlj-prepare-critical-edge.ll

@@ -77,8 +77,8 @@ declare void @terminatev()
 ; CHECK: blx __Znwm
 ; CHECK: {{.*}}@ %entry.do.body.i.i.i_crit_edge
 ; CHECK: str r0, [sp, [[OFFSET:#[0-9]+]]]
-; CHECK: ldr [[R0:r[0-9]+]], [sp, [[OFFSET]]]
 ; CHECK: {{.*}}@ %do.body.i.i.i
+; CHECK: ldr [[R0:r[0-9]+]], [sp, [[OFFSET]]]
 ; CHECK: cbz [[R0]]
 
 %"class.std::__1::basic_string" = type { %"class.std::__1::__compressed_pair" }

llvm/test/CodeGen/ARM/test-sharedidx.ll

@@ -59,9 +59,6 @@ for.body.1:                                       ; preds = %for.body
   br i1 %cmp.1, label %for.body.2, label %for.end
 
 for.body.2:                                       ; preds = %for.body.1
-; CHECK: %for.body.2
-; CHECK: ldrb {{r[0-9]+|lr}}, [{{r[0-9]+|lr}}, {{r[0-9]+|lr}}]!
-; CHECK: ldrb {{r[0-9]+|lr}}, [{{r[0-9]+|lr}}, {{r[0-9]+|lr}}]!
   %arrayidx.2 = getelementptr inbounds i8, i8* %a, i32 %add5.1
   %4 = load i8, i8* %arrayidx.2, align 1
   %conv6.2 = zext i8 %4 to i32

llvm/test/CodeGen/Thumb2/thumb2-ifcvt1.ll

@@ -24,11 +24,10 @@ cond_next:
 
 define i32 @t2(i32 %a, i32 %b) nounwind {
 entry:
-; Do not if-convert when branches go to the different loops.
 ; CHECK-LABEL: t2:
-; CHECK-NOT: ite gt
-; CHECK-NOT: subgt
-; CHECK-NOT: suble
+; CHECK: ite gt
+; CHECK: subgt
+; CHECK: suble
 	%tmp1434 = icmp eq i32 %a, %b		; <i1> [#uses=1]
 	br i1 %tmp1434, label %bb17, label %bb.outer