[PATCH] D26221: ScheduleDAGInstrs: Ignore dependencies of constant physregs

[Matthias Braun via llvm-commits]

MatzeB created this revision.
MatzeB added reviewers: atrick, jonpa, gberry, qcolombet.
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There is no need to track dependencies for constant physregs, as they don't change their value no matter in what order you read/write to them.


Repository:
  rL LLVM

https://reviews.llvm.org/D26221

Files:
  lib/CodeGen/ScheduleDAGInstrs.cpp
  test/CodeGen/AArch64/machine_cse_impdef_killflags.ll
  test/CodeGen/AArch64/scheduledag-constreg.mir
  test/CodeGen/AArch64/subs-to-sub-opt.ll


Index: test/CodeGen/AArch64/subs-to-sub-opt.ll
===================================================================
--- test/CodeGen/AArch64/subs-to-sub-opt.ll
+++ test/CodeGen/AArch64/subs-to-sub-opt.ll
@@ -7,7 +7,7 @@
 define i32 @test01() nounwind {
 ; CHECK: ldrb {{.*}}
 ; CHECK-NEXT: ldrb {{.*}}
-; CHECK-NEXT: sub {{.*}}
+; CHECK: sub {{.*}}
 ; CHECK-NEXT: cmn {{.*}}
 entry:
   %0 = load i8, i8* @a, align 1
Index: test/CodeGen/AArch64/scheduledag-constreg.mir
===================================================================
--- /dev/null
+++ test/CodeGen/AArch64/scheduledag-constreg.mir
@@ -0,0 +1,29 @@
+# RUN: llc -o /dev/null %s -mtriple=aarch64-- -run-pass=machine-scheduler -enable-misched -debug-only=misched 2>&1 | FileCheck %s
+# REQUIRES: asserts
+--- |
+  define void @func() { ret void }
+...
+---
+# Check that the instructions are not dependent on each other, even though
+# they all read/write to the zero register.
+# CHECK-LABEL: MI Scheduling
+# CHECK: SU(0): %WZR<def,dead> = SUBSWri %W1, 0, 0, %NZCV<imp-def,dead>
+# CHECK: # succs left : 0
+# CHECK-NOT: Successors:
+# CHECK: SU(1): %W2<def> = COPY %WZR
+# CHECK: # succs left : 0
+# CHECK-NOT: Successors:
+# CHECK: SU(2): %WZR<def,dead> = SUBSWri %W3, 0, 0, %NZCV<imp-def,dead>
+# CHECK: # succs left : 0
+# CHECK-NOT: Successors:
+# CHECK: SU(3): %W4<def> = COPY %WZR
+# CHECK: # succs left : 0
+# CHECK-NOT: Successors:
+name: func
+body: |
+  bb.0:
+    dead %wzr = SUBSWri %w1, 0, 0, implicit-def dead %nzcv
+    %w2 = COPY %wzr
+    dead %wzr = SUBSWri %w3, 0, 0, implicit-def dead %nzcv
+    %w4 = COPY %wzr
+...
Index: test/CodeGen/AArch64/machine_cse_impdef_killflags.ll
===================================================================
--- test/CodeGen/AArch64/machine_cse_impdef_killflags.ll
+++ test/CodeGen/AArch64/machine_cse_impdef_killflags.ll
@@ -8,8 +8,8 @@
 ; CHECK-DAG:  orr    [[REG1:w[0-9]+]], wzr, #0x1
 ; CHECK-DAG:  orr    [[REG2:x[0-9]+]], xzr, #0x2
 ; CHECK-DAG:  orr    [[REG3:x[0-9]+]], xzr, #0x3
-; CHECK:      cmp    x0, #0
-; CHECK-DAG:  csel   w[[SELECT_WREG_1:[0-9]+]], wzr, [[REG1]], ne
+; CHECK-DAG:  cmp    x0, #0
+; CHECK:  csel   w[[SELECT_WREG_1:[0-9]+]], wzr, [[REG1]], ne
 ; CHECK-DAG:  csel   [[SELECT_XREG_2:x[0-9]+]], [[REG2]], [[REG3]], ne
 ; CHECK:      ubfx   [[SELECT_XREG_1:x[0-9]+]], x[[SELECT_WREG_1]], #0, #32
 ; CHECK-NEXT: add    x0, [[SELECT_XREG_2]], [[SELECT_XREG_1]]
Index: lib/CodeGen/ScheduleDAGInstrs.cpp
===================================================================
--- lib/CodeGen/ScheduleDAGInstrs.cpp
+++ lib/CodeGen/ScheduleDAGInstrs.cpp
@@ -323,6 +323,9 @@
   MachineInstr *MI = SU->getInstr();
   MachineOperand &MO = MI->getOperand(OperIdx);
   unsigned Reg = MO.getReg();
+  // We do not need to track any dependencies for constant registers.
+  if (MRI.isConstantPhysReg(Reg))
+    return;
 
   // Optionally add output and anti dependencies. For anti
   // dependencies we use a latency of 0 because for a multi-issue


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