[RAGreedy] Enable -consider-local-interval-cost for AArch64

Summary:
The greedy register allocator occasionally decides to insert a large number of
unnecessary copies, see below for an example.  The -consider-local-interval-cost
option (which X86 already enables by default) fixes this.  We enable this option
for AArch64 only after receiving feedback that this change is not beneficial for
PowerPC.

We evaluated the impact of this change on compile time, code size and
performance benchmarks.

This option has a small impact on compile time, measured on CTMark. A 0.1%
geomean regression on -O1 and -O2, and 0.2% geomean for -O3, with at most 0.5%
on individual benchmarks.

The effect on both code size and performance on AArch64 for the LLVM test suite
is nil on the geomean with individual outliers (ignoring short exec_times)
between:

                 best     worst
  size..text     -3.3%    +0.0%
  exec_time      -5.8%    +2.3%

On SPEC CPU® 2017 (compiled for AArch64) there is a minor reduction (-0.2% at
most) in code size on some benchmarks, with a tiny movement (-0.01%) on the
geomean.  Neither intrate nor fprate show any change in performance.

This patch makes the following changes.

- For the AArch64 target, enableAdvancedRASplitCost() now returns true.

- Ensures that -consider-local-interval-cost=false can disable the new
  behaviour if necessary.

This matrix multiply example:

   $ cat test.c
   long A[8][8];
   long B[8][8];
   long C[8][8];

   void run_test() {
     for (int k = 0; k < 8; k++) {
       for (int i = 0; i < 8; i++) {
	 for (int j = 0; j < 8; j++) {
	   C[i][j] += A[i][k] * B[k][j];
	 }
       }
     }
   }

results in the following generated code on AArch64:

  $ clang --target=aarch64-arm-none-eabi -O3 -S test.c -o -
  [...]
                                        // %for.cond1.preheader
                                        // =>This Inner Loop Header: Depth=1
        add     x14, x11, x9
        str     q0, [sp, #16]           // 16-byte Folded Spill
        ldr     q0, [x14]
        mov     v2.16b, v15.16b
        mov     v15.16b, v14.16b
        mov     v14.16b, v13.16b
        mov     v13.16b, v12.16b
        mov     v12.16b, v11.16b
        mov     v11.16b, v10.16b
        mov     v10.16b, v9.16b
        mov     v9.16b, v8.16b
        mov     v8.16b, v31.16b
        mov     v31.16b, v30.16b
        mov     v30.16b, v29.16b
        mov     v29.16b, v28.16b
        mov     v28.16b, v27.16b
        mov     v27.16b, v26.16b
        mov     v26.16b, v25.16b
        mov     v25.16b, v24.16b
        mov     v24.16b, v23.16b
        mov     v23.16b, v22.16b
        mov     v22.16b, v21.16b
        mov     v21.16b, v20.16b
        mov     v20.16b, v19.16b
        mov     v19.16b, v18.16b
        mov     v18.16b, v17.16b
        mov     v17.16b, v16.16b
        mov     v16.16b, v7.16b
        mov     v7.16b, v6.16b
        mov     v6.16b, v5.16b
        mov     v5.16b, v4.16b
        mov     v4.16b, v3.16b
        mov     v3.16b, v1.16b
        mov     x12, v0.d[1]
        fmov    x15, d0
        ldp     q1, q0, [x14, #16]
        ldur    x1, [x10, #-256]
        ldur    x2, [x10, #-192]
        add     x9, x9, #64             // =64
        mov     x13, v1.d[1]
        fmov    x16, d1
        ldr     q1, [x14, #48]
        mul     x3, x15, x1
        mov     x14, v0.d[1]
        fmov    x17, d0
        mov     x18, v1.d[1]
        fmov    x0, d1
        mov     v1.16b, v3.16b
        mov     v3.16b, v4.16b
        mov     v4.16b, v5.16b
        mov     v5.16b, v6.16b
        mov     v6.16b, v7.16b
        mov     v7.16b, v16.16b
        mov     v16.16b, v17.16b
        mov     v17.16b, v18.16b
        mov     v18.16b, v19.16b
        mov     v19.16b, v20.16b
        mov     v20.16b, v21.16b
        mov     v21.16b, v22.16b
        mov     v22.16b, v23.16b
        mov     v23.16b, v24.16b
        mov     v24.16b, v25.16b
        mov     v25.16b, v26.16b
        mov     v26.16b, v27.16b
        mov     v27.16b, v28.16b
        mov     v28.16b, v29.16b
        mov     v29.16b, v30.16b
        mov     v30.16b, v31.16b
        mov     v31.16b, v8.16b
        mov     v8.16b, v9.16b
        mov     v9.16b, v10.16b
        mov     v10.16b, v11.16b
        mov     v11.16b, v12.16b
        mov     v12.16b, v13.16b
        mov     v13.16b, v14.16b
        mov     v14.16b, v15.16b
        mov     v15.16b, v2.16b
        ldr     q2, [sp]                // 16-byte Folded Reload
        fmov    d0, x3
        mul     x3, x12, x1
  [...]

With -consider-local-interval-cost the same section of code results in the
following:

  $ clang --target=aarch64-arm-none-eabi -mllvm -consider-local-interval-cost -O3 -S test.c -o -
  [...]
  .LBB0_1:                              // %for.cond1.preheader
                                        // =>This Inner Loop Header: Depth=1
        add     x14, x11, x9
        ldp     q0, q1, [x14]
        ldur    x1, [x10, #-256]
        ldur    x2, [x10, #-192]
        add     x9, x9, #64             // =64
        mov     x12, v0.d[1]
        fmov    x15, d0
        mov     x13, v1.d[1]
        fmov    x16, d1
        ldp     q0, q1, [x14, #32]
        mul     x3, x15, x1
        cmp     x9, #512                // =512
        mov     x14, v0.d[1]
        fmov    x17, d0
        fmov    d0, x3
        mul     x3, x12, x1
  [...]

Reviewers: SjoerdMeijer, samparker, dmgreen, qcolombet

Reviewed By: dmgreen

Subscribers: ZhangKang, jsji, wuzish, ppc-slack, lkail, steven.zhang, MatzeB, qcolombet, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69437

lib/CodeGen/RegAllocGreedy.cpp

@@ -3225,8 +3225,10 @@ bool RAGreedy::runOnMachineFunction(MachineFunction &mf) {
                         MF->getSubtarget().enableRALocalReassignment(
                             MF->getTarget().getOptLevel());
 
-  EnableAdvancedRASplitCost = ConsiderLocalIntervalCost ||
+  EnableAdvancedRASplitCost =
-                              MF->getSubtarget().enableAdvancedRASplitCost();
+      ConsiderLocalIntervalCost.getNumOccurrences()
+          ? ConsiderLocalIntervalCost
+          : MF->getSubtarget().enableAdvancedRASplitCost();
 
   if (VerifyEnabled)
     MF->verify(this, "Before greedy register allocator");

lib/Target/AArch64/AArch64Subtarget.h

@@ -475,6 +475,8 @@ public:
 
   bool enableEarlyIfConversion() const override;
 
+  bool enableAdvancedRASplitCost() const override { return true; }
+
   std::unique_ptr<PBQPRAConstraint> getCustomPBQPConstraints() const override;
 
   bool isCallingConvWin64(CallingConv::ID CC) const {

test/CodeGen/AArch64/ragreedy-local-interval-cost.ll

@@ -26,9 +26,10 @@ define dso_local void @run_test() local_unnamed_addr #0 {
 ; CHECK-NEXT:    adrp x11, A
 ; CHECK-NEXT:    mov x8, xzr
 ; CHECK-NEXT:    mov x9, xzr
-; CHECK-NEXT:    movi v14.2d, #0000000000000000
+; CHECK-NEXT:    movi v0.2d, #0000000000000000
 ; CHECK-NEXT:    add x10, x10, :lo12:B+48
 ; CHECK-NEXT:    add x11, x11, :lo12:A
+; CHECK-NEXT:    str q0, [sp] // 16-byte Folded Spill
 ; CHECK-NEXT:    // implicit-def: $q1
 ; CHECK-NEXT:    // implicit-def: $q2
 ; CHECK-NEXT:    // implicit-def: $q3
@@ -47,8 +48,8 @@ define dso_local void @run_test() local_unnamed_addr #0 {
 ; CHECK-NEXT:    // implicit-def: $q24
 ; CHECK-NEXT:    // implicit-def: $q25
 ; CHECK-NEXT:    // implicit-def: $q26
-; CHECK-NEXT:    // implicit-def: $q28
 ; CHECK-NEXT:    // implicit-def: $q27
+; CHECK-NEXT:    // implicit-def: $q28
 ; CHECK-NEXT:    // implicit-def: $q29
 ; CHECK-NEXT:    // implicit-def: $q30
 ; CHECK-NEXT:    // implicit-def: $q31
@@ -60,78 +61,20 @@ define dso_local void @run_test() local_unnamed_addr #0 {
 ; CHECK-NEXT:    // implicit-def: $q13
 ; CHECK-NEXT:  .LBB0_1: // %for.cond1.preheader
 ; CHECK-NEXT:    // =>This Inner Loop Header: Depth=1
-; CHECK-NEXT:    str q13, [sp] // 16-byte Folded Spill
 ; CHECK-NEXT:    mov x12, xzr
 ; CHECK-NEXT:    ldr q15, [x8]
-; CHECK-NEXT:    mov v13.16b, v12.16b
-; CHECK-NEXT:    mov v12.16b, v11.16b
-; CHECK-NEXT:    mov v11.16b, v10.16b
-; CHECK-NEXT:    mov v10.16b, v9.16b
-; CHECK-NEXT:    mov v9.16b, v8.16b
-; CHECK-NEXT:    mov v8.16b, v31.16b
-; CHECK-NEXT:    mov v31.16b, v30.16b
-; CHECK-NEXT:    mov v30.16b, v29.16b
-; CHECK-NEXT:    mov v29.16b, v27.16b
-; CHECK-NEXT:    mov v27.16b, v26.16b
-; CHECK-NEXT:    mov v26.16b, v25.16b
-; CHECK-NEXT:    mov v25.16b, v24.16b
-; CHECK-NEXT:    mov v24.16b, v23.16b
-; CHECK-NEXT:    mov v23.16b, v22.16b
-; CHECK-NEXT:    mov v22.16b, v21.16b
-; CHECK-NEXT:    mov v21.16b, v20.16b
-; CHECK-NEXT:    mov v20.16b, v19.16b
-; CHECK-NEXT:    mov v19.16b, v18.16b
-; CHECK-NEXT:    mov v18.16b, v17.16b
-; CHECK-NEXT:    mov v17.16b, v16.16b
-; CHECK-NEXT:    mov v16.16b, v7.16b
-; CHECK-NEXT:    mov v7.16b, v6.16b
-; CHECK-NEXT:    mov v6.16b, v5.16b
-; CHECK-NEXT:    mov v5.16b, v4.16b
-; CHECK-NEXT:    mov v4.16b, v3.16b
-; CHECK-NEXT:    mov v3.16b, v2.16b
-; CHECK-NEXT:    mov v2.16b, v1.16b
-; CHECK-NEXT:    mov v1.16b, v14.16b
 ; CHECK-NEXT:    ldr q14, [x12]
 ; CHECK-NEXT:    ldr q0, [x10], #64
 ; CHECK-NEXT:    ldr x18, [x12]
 ; CHECK-NEXT:    fmov x15, d15
 ; CHECK-NEXT:    mov x14, v15.d[1]
-; CHECK-NEXT:    mov x12, v14.d[1]
-; CHECK-NEXT:    mul x1, x15, x18
 ; CHECK-NEXT:    fmov x13, d14
-; CHECK-NEXT:    mov v14.16b, v1.16b
+; CHECK-NEXT:    mul x1, x15, x18
-; CHECK-NEXT:    mov v1.16b, v2.16b
-; CHECK-NEXT:    mov v2.16b, v3.16b
-; CHECK-NEXT:    mov v3.16b, v4.16b
-; CHECK-NEXT:    mov v4.16b, v5.16b
-; CHECK-NEXT:    mov v5.16b, v6.16b
-; CHECK-NEXT:    mov v6.16b, v7.16b
-; CHECK-NEXT:    mov v7.16b, v16.16b
-; CHECK-NEXT:    mov v16.16b, v17.16b
-; CHECK-NEXT:    mov v17.16b, v18.16b
-; CHECK-NEXT:    mov v18.16b, v19.16b
-; CHECK-NEXT:    mov v19.16b, v20.16b
-; CHECK-NEXT:    mov v20.16b, v21.16b
-; CHECK-NEXT:    mov v21.16b, v22.16b
-; CHECK-NEXT:    mov v22.16b, v23.16b
-; CHECK-NEXT:    mov v23.16b, v24.16b
-; CHECK-NEXT:    mov v24.16b, v25.16b
-; CHECK-NEXT:    mov v25.16b, v26.16b
-; CHECK-NEXT:    mov v26.16b, v27.16b
-; CHECK-NEXT:    mov v27.16b, v29.16b
-; CHECK-NEXT:    mov v29.16b, v30.16b
-; CHECK-NEXT:    mov v30.16b, v31.16b
-; CHECK-NEXT:    mov v31.16b, v8.16b
-; CHECK-NEXT:    mov v8.16b, v9.16b
-; CHECK-NEXT:    mov v9.16b, v10.16b
-; CHECK-NEXT:    mov v10.16b, v11.16b
-; CHECK-NEXT:    mov v11.16b, v12.16b
-; CHECK-NEXT:    mov v12.16b, v13.16b
-; CHECK-NEXT:    ldr q13, [sp] // 16-byte Folded Reload
 ; CHECK-NEXT:    mov x16, v0.d[1]
 ; CHECK-NEXT:    fmov x17, d0
 ; CHECK-NEXT:    fmov d0, x1
 ; CHECK-NEXT:    mul x1, x14, x18
+; CHECK-NEXT:    mov x12, v14.d[1]
 ; CHECK-NEXT:    ldr x0, [x8]
 ; CHECK-NEXT:    mov v0.d[1], x1
 ; CHECK-NEXT:    mul x1, x13, x18
@@ -144,6 +87,7 @@ define dso_local void @run_test() local_unnamed_addr #0 {
 ; CHECK-NEXT:    add v11.2d, v11.2d, v0.2d
 ; CHECK-NEXT:    fmov d0, x1
 ; CHECK-NEXT:    mul x18, x16, x18
+; CHECK-NEXT:    ldr q14, [sp] // 16-byte Folded Reload
 ; CHECK-NEXT:    mov v0.d[1], x18
 ; CHECK-NEXT:    mul x18, x15, x0
 ; CHECK-NEXT:    add x1, x11, x8
@@ -194,28 +138,29 @@ define dso_local void @run_test() local_unnamed_addr #0 {
 ; CHECK-NEXT:    mul x17, x17, x1
 ; CHECK-NEXT:    mov v0.d[1], x12
 ; CHECK-NEXT:    mul x16, x16, x1
-; CHECK-NEXT:    add v27.2d, v27.2d, v0.2d
+; CHECK-NEXT:    add v28.2d, v28.2d, v0.2d
 ; CHECK-NEXT:    fmov d0, x17
 ; CHECK-NEXT:    mov v0.d[1], x16
 ; CHECK-NEXT:    add x8, x8, #8 // =8
-; CHECK-NEXT:    add v28.2d, v28.2d, v0.2d
+; CHECK-NEXT:    add v27.2d, v27.2d, v0.2d
 ; CHECK-NEXT:    cmp x8, #64 // =64
 ; CHECK-NEXT:    add x9, x9, #1 // =1
+; CHECK-NEXT:    str q14, [sp] // 16-byte Folded Spill
 ; CHECK-NEXT:    b.ne .LBB0_1
 ; CHECK-NEXT:  // %bb.2: // %for.cond.cleanup
 ; CHECK-NEXT:    adrp x8, C
 ; CHECK-NEXT:    add x8, x8, :lo12:C
+; CHECK-NEXT:    ldr q0, [sp] // 16-byte Folded Reload
 ; CHECK-NEXT:    stp q13, q12, [x8]
 ; CHECK-NEXT:    stp q11, q10, [x8, #32]
 ; CHECK-NEXT:    stp q9, q8, [x8, #64]
-; CHECK-NEXT:    stp q14, q2, [x8, #464]
 ; CHECK-NEXT:    ldp d9, d8, [sp, #64] // 16-byte Folded Reload
 ; CHECK-NEXT:    ldp d11, d10, [sp, #48] // 16-byte Folded Reload
 ; CHECK-NEXT:    ldp d13, d12, [sp, #32] // 16-byte Folded Reload
 ; CHECK-NEXT:    ldp d15, d14, [sp, #16] // 16-byte Folded Reload
 ; CHECK-NEXT:    stp q31, q30, [x8, #96]
-; CHECK-NEXT:    stp q29, q27, [x8, #144]
+; CHECK-NEXT:    stp q29, q28, [x8, #144]
-; CHECK-NEXT:    stp q28, q26, [x8, #176]
+; CHECK-NEXT:    stp q27, q26, [x8, #176]
 ; CHECK-NEXT:    str q25, [x8, #208]
 ; CHECK-NEXT:    stp q24, q23, [x8, #240]
 ; CHECK-NEXT:    stp q22, q21, [x8, #272]
@@ -224,6 +169,7 @@ define dso_local void @run_test() local_unnamed_addr #0 {
 ; CHECK-NEXT:    stp q16, q7, [x8, #368]
 ; CHECK-NEXT:    stp q6, q5, [x8, #400]
 ; CHECK-NEXT:    stp q4, q3, [x8, #432]
+; CHECK-NEXT:    stp q0, q2, [x8, #464]
 ; CHECK-NEXT:    str q1, [x8, #496]
 ; CHECK-NEXT:    add sp, sp, #80 // =80
 ; CHECK-NEXT:    ret