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Scaling up values in ARMBaseInstrInfo::isProfitableToIfCvt() before they are scaled by a probability to avoid precision issue.

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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
This commit is contained in:
Cong Hou 2015-09-18 18:19:40 +00:00
parent 9b1349ccd2
commit 7856c96129
6 changed files with 28 additions and 22 deletions
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25
lib/Target/ARM/ARMBaseInstrInfo.cpp
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@ -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;
// 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) <= UnpredCost;
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

7
test/CodeGen/ARM/2013-10-11-select-stalls.ll
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@ -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)

6
test/CodeGen/ARM/ifcvt4.ll
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@ -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]

2
test/CodeGen/ARM/sjlj-prepare-critical-edge.ll
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@ -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" }

3
test/CodeGen/ARM/test-sharedidx.ll
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@ -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

7
test/CodeGen/Thumb2/thumb2-ifcvt1.ll
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@ -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