[PATCH] D19349: MachineScheduler: Limit the size of the ready list.

[Matthias Braun via llvm-commits]

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Avoid quadratic complexity in unusually large basic blocks by limiting
the size of the ready lists.

Measuring llvm test-suite for SPEC2000 shows that the ready queue size never exceed 200 on X86 and only exceeds the 256 limit 3 times on AArch64 (with benign/no changes).

Repository:
  rL LLVM

http://reviews.llvm.org/D19349

Files:
  lib/CodeGen/MachineScheduler.cpp
  test/CodeGen/AArch64/arm64-misched-basic-A53.ll
  test/CodeGen/X86/misched-matmul.ll

Index: test/CodeGen/X86/misched-matmul.ll
===================================================================
--- test/CodeGen/X86/misched-matmul.ll
+++ test/CodeGen/X86/misched-matmul.ll
@@ -1,5 +1,6 @@
 ; REQUIRES: asserts
 ; RUN: llc < %s -march=x86-64 -mcpu=core2 -pre-RA-sched=source -enable-misched -stats 2>&1 | FileCheck %s
+; RUN: llc < %s -march=x86-64 -mcpu=core2 -pre-RA-sched=source -enable-misched -stats -misched-limit=8 2>&1 | FileCheck %s
 ;
 ; Verify that register pressure heuristics are working in MachineScheduler.
 ;
Index: test/CodeGen/AArch64/arm64-misched-basic-A53.ll
===================================================================
--- test/CodeGen/AArch64/arm64-misched-basic-A53.ll
+++ test/CodeGen/AArch64/arm64-misched-basic-A53.ll
@@ -1,5 +1,6 @@
 ; REQUIRES: asserts
 ; RUN: llc < %s -mtriple=arm64-linux-gnu -mcpu=cortex-a53 -pre-RA-sched=source -enable-misched -verify-misched -debug-only=misched -o - 2>&1 > /dev/null | FileCheck %s
+; RUN: llc < %s -mtriple=arm64-linux-gnu -mcpu=cortex-a53 -pre-RA-sched=source -enable-misched -verify-misched -debug-only=misched -o - -misched-limit=2 2>&1 > /dev/null | FileCheck %s
 ;
 ; The Cortex-A53 machine model will cause the MADD instruction to be scheduled
 ; much higher than the ADD instructions in order to hide latency. When not
Index: lib/CodeGen/MachineScheduler.cpp
===================================================================
--- lib/CodeGen/MachineScheduler.cpp
+++ lib/CodeGen/MachineScheduler.cpp
@@ -64,6 +64,11 @@
 static bool ViewMISchedDAGs = false;
 #endif // NDEBUG
 
+/// Avoid quadratic complexity in unusually large basic blocks by limiting the
+/// size of the ready lists.
+static cl::opt<unsigned> ReadyListLimit("misched-limit", cl::Hidden,
+  cl::desc("Limit ready list to N instructions"), cl::init(256));
+
 static cl::opt<bool> EnableRegPressure("misched-regpressure", cl::Hidden,
   cl::desc("Enable register pressure scheduling."), cl::init(true));
 
@@ -1956,7 +1961,8 @@
   // Check for interlocks first. For the purpose of other heuristics, an
   // instruction that cannot issue appears as if it's not in the ReadyQueue.
   bool IsBuffered = SchedModel->getMicroOpBufferSize() != 0;
-  if ((!IsBuffered && ReadyCycle > CurrCycle) || checkHazard(SU))
+  if ((!IsBuffered && ReadyCycle > CurrCycle) || checkHazard(SU) ||
+      Available.size() >= ReadyListLimit)
     Pending.push(SU);
   else
     Available.push(SU);


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