[llvm] r248018 - Scaling up values in ARMBaseInstrInfo::isProfitableToIfCvt() before they are scaled by a probability to avoid precision issue.
Cong Hou via llvm-commits
llvm-commits at lists.llvm.org
Fri Sep 18 11:19:41 PDT 2015
Author: conghou
Date: Fri Sep 18 13:19:40 2015
New Revision: 248018
URL: http://llvm.org/viewvc/llvm-project?rev=248018&view=rev
Log:
Scaling up values in ARMBaseInstrInfo::isProfitableToIfCvt() before they 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
Modified:
llvm/trunk/lib/Target/ARM/ARMBaseInstrInfo.cpp
llvm/trunk/test/CodeGen/ARM/2013-10-11-select-stalls.ll
llvm/trunk/test/CodeGen/ARM/ifcvt4.ll
llvm/trunk/test/CodeGen/ARM/sjlj-prepare-critical-edge.ll
llvm/trunk/test/CodeGen/ARM/test-sharedidx.ll
llvm/trunk/test/CodeGen/Thumb2/thumb2-ifcvt1.ll
Modified: llvm/trunk/lib/Target/ARM/ARMBaseInstrInfo.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/ARM/ARMBaseInstrInfo.cpp?rev=248018&r1=248017&r2=248018&view=diff
==============================================================================
--- llvm/trunk/lib/Target/ARM/ARMBaseInstrInfo.cpp (original)
+++ llvm/trunk/lib/Target/ARM/ARMBaseInstrInfo.cpp Fri Sep 18 13:19:40 2015
@@ -1670,11 +1670,14 @@ isProfitableToIfCvt(MachineBasicBlock &M
}
// 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 &T
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
Modified: llvm/trunk/test/CodeGen/ARM/2013-10-11-select-stalls.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/ARM/2013-10-11-select-stalls.ll?rev=248018&r1=248017&r2=248018&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/ARM/2013-10-11-select-stalls.ll (original)
+++ llvm/trunk/test/CodeGen/ARM/2013-10-11-select-stalls.ll Fri Sep 18 13:19:40 2015
@@ -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)
Modified: llvm/trunk/test/CodeGen/ARM/ifcvt4.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/ARM/ifcvt4.ll?rev=248018&r1=248017&r2=248018&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/ARM/ifcvt4.ll (original)
+++ llvm/trunk/test/CodeGen/ARM/ifcvt4.ll Fri Sep 18 13:19:40 2015
@@ -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]
Modified: llvm/trunk/test/CodeGen/ARM/sjlj-prepare-critical-edge.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/ARM/sjlj-prepare-critical-edge.ll?rev=248018&r1=248017&r2=248018&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/ARM/sjlj-prepare-critical-edge.ll (original)
+++ llvm/trunk/test/CodeGen/ARM/sjlj-prepare-critical-edge.ll Fri Sep 18 13:19:40 2015
@@ -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" }
Modified: llvm/trunk/test/CodeGen/ARM/test-sharedidx.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/ARM/test-sharedidx.ll?rev=248018&r1=248017&r2=248018&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/ARM/test-sharedidx.ll (original)
+++ llvm/trunk/test/CodeGen/ARM/test-sharedidx.ll Fri Sep 18 13:19:40 2015
@@ -59,9 +59,6 @@ for.body.1:
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
Modified: llvm/trunk/test/CodeGen/Thumb2/thumb2-ifcvt1.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/Thumb2/thumb2-ifcvt1.ll?rev=248018&r1=248017&r2=248018&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/Thumb2/thumb2-ifcvt1.ll (original)
+++ llvm/trunk/test/CodeGen/Thumb2/thumb2-ifcvt1.ll Fri Sep 18 13:19:40 2015
@@ -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
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