From b329e0728b3eda7a1c754931e5c174b2b7ae51b3 Mon Sep 17 00:00:00 2001
From: Guozhi Wei <carrot@google.com>
Date: Tue, 10 Sep 2019 17:25:11 +0000
Subject: [PATCH] [BPI] Adjust the probability for floating point unordered
 comparison

Since NaN is very rare in normal programs, so the probability for floating point unordered comparison should be extremely small. Current probability is 3/8, it is too large, this patch changes it to a tiny number.

Differential Revision: https://reviews.llvm.org/D65303

llvm-svn: 371541
---
 llvm/lib/Analysis/BranchProbabilityInfo.cpp      | 16 +++++++--
 llvm/test/Analysis/BranchProbabilityInfo/fcmp.ll | 41 ++++++++++++++++++++++++
 llvm/test/CodeGen/SystemZ/call-05.ll             |  3 +-
 3 files changed, 57 insertions(+), 3 deletions(-)
 create mode 100644 llvm/test/Analysis/BranchProbabilityInfo/fcmp.ll

diff --git a/llvm/lib/Analysis/BranchProbabilityInfo.cpp b/llvm/lib/Analysis/BranchProbabilityInfo.cpp
index 65c9cf2..a06ee09 100644
--- a/llvm/lib/Analysis/BranchProbabilityInfo.cpp
+++ b/llvm/lib/Analysis/BranchProbabilityInfo.cpp
@@ -118,6 +118,13 @@ static const uint32_t ZH_NONTAKEN_WEIGHT = 12;
 static const uint32_t FPH_TAKEN_WEIGHT = 20;
 static const uint32_t FPH_NONTAKEN_WEIGHT = 12;
 
+/// This is the probability for an ordered floating point comparison.
+static const uint32_t FPH_ORD_WEIGHT = 1024 * 1024 - 1;
+/// This is the probability for an unordered floating point comparison, it means
+/// one or two of the operands are NaN. Usually it is used to test for an
+/// exceptional case, so the result is unlikely.
+static const uint32_t FPH_UNO_WEIGHT = 1;
+
 /// Invoke-terminating normal branch taken weight
 ///
 /// This is the weight for branching to the normal destination of an invoke
@@ -778,6 +785,8 @@ bool BranchProbabilityInfo::calcFloatingPointHeuristics(const BasicBlock *BB) {
   if (!FCmp)
     return false;
 
+  uint32_t TakenWeight = FPH_TAKEN_WEIGHT;
+  uint32_t NontakenWeight = FPH_NONTAKEN_WEIGHT;
   bool isProb;
   if (FCmp->isEquality()) {
     // f1 == f2 -> Unlikely
@@ -786,9 +795,13 @@ bool BranchProbabilityInfo::calcFloatingPointHeuristics(const BasicBlock *BB) {
   } else if (FCmp->getPredicate() == FCmpInst::FCMP_ORD) {
     // !isnan -> Likely
     isProb = true;
+    TakenWeight = FPH_ORD_WEIGHT;
+    NontakenWeight = FPH_UNO_WEIGHT;
   } else if (FCmp->getPredicate() == FCmpInst::FCMP_UNO) {
     // isnan -> Unlikely
     isProb = false;
+    TakenWeight = FPH_ORD_WEIGHT;
+    NontakenWeight = FPH_UNO_WEIGHT;
   } else {
     return false;
   }
@@ -798,8 +811,7 @@ bool BranchProbabilityInfo::calcFloatingPointHeuristics(const BasicBlock *BB) {
   if (!isProb)
     std::swap(TakenIdx, NonTakenIdx);
 
-  BranchProbability TakenProb(FPH_TAKEN_WEIGHT,
-                              FPH_TAKEN_WEIGHT + FPH_NONTAKEN_WEIGHT);
+  BranchProbability TakenProb(TakenWeight, TakenWeight + NontakenWeight);
   setEdgeProbability(BB, TakenIdx, TakenProb);
   setEdgeProbability(BB, NonTakenIdx, TakenProb.getCompl());
   return true;
diff --git a/llvm/test/Analysis/BranchProbabilityInfo/fcmp.ll b/llvm/test/Analysis/BranchProbabilityInfo/fcmp.ll
new file mode 100644
index 0000000..8089916
--- /dev/null
+++ b/llvm/test/Analysis/BranchProbabilityInfo/fcmp.ll
@@ -0,0 +1,41 @@
+; RUN: opt < %s -analyze -branch-prob | FileCheck %s
+
+; This function tests the floating point unorder comparison. The probability
+; of NaN should be extremely small.
+; CHECK: Printing analysis 'Branch Probability Analysis' for function 'uno'
+; CHECK:  edge  -> a probability is 0x00000800 / 0x80000000 = 0.00%
+; CHECK:  edge  -> b probability is 0x7ffff800 / 0x80000000 = 100.00% [HOT edge]
+
+define void @uno(float %val1, float %val2) {
+  %cond = fcmp uno float %val1, %val2
+  br i1 %cond, label %a, label %b
+
+a:
+  call void @fa()
+  ret void
+
+b:
+  call void @fb()
+  ret void
+}
+
+; This function tests the floating point order comparison.
+; CHECK: Printing analysis 'Branch Probability Analysis' for function 'ord'
+; CHECK:  edge  -> a probability is 0x7ffff800 / 0x80000000 = 100.00% [HOT edge]
+; CHECK:  edge  -> b probability is 0x00000800 / 0x80000000 = 0.00%
+
+define void @ord(float %val1, float %val2) {
+  %cond = fcmp ord float %val1, %val2
+  br i1 %cond, label %a, label %b
+
+a:
+  call void @fa()
+  ret void
+
+b:
+  call void @fb()
+  ret void
+}
+
+declare void @fa()
+declare void @fb()
diff --git a/llvm/test/CodeGen/SystemZ/call-05.ll b/llvm/test/CodeGen/SystemZ/call-05.ll
index 1570453..b958c0b 100644
--- a/llvm/test/CodeGen/SystemZ/call-05.ll
+++ b/llvm/test/CodeGen/SystemZ/call-05.ll
@@ -451,8 +451,9 @@ b:
 define void @f25(float %val1, float %val2) {
 ; CHECK-LABEL: f25:
 ; CHECK: cebr %f0, %f2
-; CHECK: bor %r1
+; CHECK: jo
 ; CHECK: br %r14
+; CHECK: br %r1
   %fun_a = load volatile void() *, void()** @fun_a;
   %cond = fcmp uno float %val1, %val2;
   br i1 %cond, label %a, label %b;
-- 
2.7.4