[RegAllocEvictAdvisor][NFC] Add minimum weight ratio heuristic.

Do not evict a live range if eviction will break existing hint without
satisfying a new one and the ratio of weights is not big enough. The
heuristic is disabled by default.

Author: mgudim

llvm/lib/CodeGen/RegAllocEvictionAdvisor.cpp

@@ -44,6 +44,13 @@ static cl::opt<bool> EnableLocalReassignment(
              "may be compile time intensive"),
     cl::init(false));
 
+static cl::opt<float> MinWeightRatioNeededToEvictHint(
+    "min-weight-ratio-needed-to-evict-hint", cl::Hidden,
+    cl::desc("The minimum ratio of weights needed in order for the live range "
+             "with bigger weight to evict the other live range which"
+             "satisfies a hint"),
+    cl::init(1.0));
+
 namespace llvm {
 cl::opt<unsigned> EvictInterferenceCutoff(
     "regalloc-eviction-max-interference-cutoff", cl::Hidden,
@@ -156,8 +163,15 @@ bool DefaultEvictionAdvisor::shouldEvict(const LiveInterval &A, bool IsHint,
   if (CanSplit && IsHint && !BreaksHint)
     return true;
 
-  if (A.weight() > B.weight()) {
-    LLVM_DEBUG(dbgs() << "should evict: " << B << " w= " << B.weight() << '\n');
+  float AWeight = A.weight();
+  float BWeight = B.weight();
+  if (AWeight > BWeight) {
+    float WeightRatio = BWeight == 0.0 ? std::numeric_limits<float>::infinity()
+                                       : AWeight / BWeight;
+    if (CanSplit && !IsHint && BreaksHint &&
+        (WeightRatio < MinWeightRatioNeededToEvictHint))
+      return false;
+    LLVM_DEBUG(dbgs() << "should evict: " << B << " w= " << BWeight << '\n');
     return true;
   }
   return false;

llvm/test/CodeGen/RISCV/min-weight-ratio.mir

@@ -0,0 +1,76 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
+# RUN: llc %s -mtriple=riscv64 -run-pass=greedy,virtregrewriter \
+# RUN: -min-weight-ratio-needed-to-evict-hint=7.5 -o - | FileCheck %s
+
+# Due to the minimum weight heuristic (see the explanation of `MinWeightRatioNeededToEvictHint`) we are able to get rid of ALL the copies in the code below. When heuristic is disabled (-min-weight-ratio-needed-to-evict-hint=1.0) we end up with 3 copies.
+
+--- |
+  source_filename = "test.c"
+  target datalayout = "e-m:e-p:64:64-i64:64-i128:128-n32:64-S128"
+  target triple = "riscv64"
+
+  define dso_local i64 @caller(i64 %x) {
+  entry:
+    %add_ = add i64 %x, 1
+    call void @foo()
+    ret i64 %add_
+  }
+
+  declare void @foo()
+
+...
+---
+name:            caller
+alignment:       2
+tracksRegLiveness: true
+body:             |
+  bb.0.entry:
+    liveins: $x10, $x1, $x8, $x9, $x18, $x19, $x20, $x21, $x22, $x23, $x24, $x25, $x26, $x27
+
+    ; CHECK-LABEL: name: caller
+    ; CHECK: liveins: $x1, $x8, $x9, $x10, $x18, $x19, $x20, $x21, $x22, $x23, $x24, $x25, $x26, $x27
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: SD $x1, %stack.0, 0 :: (store (s64) into %stack.0)
+    ; CHECK-NEXT: renamable $x10 = ADDI killed renamable $x10, 1
+    ; CHECK-NEXT: SD killed renamable $x10, %stack.1, 0 :: (store (s64) into %stack.1)
+    ; CHECK-NEXT: ADJCALLSTACKDOWN 0, 0, implicit-def dead $x2, implicit $x2
+    ; CHECK-NEXT: PseudoCALL target-flags(riscv-call) @foo, csr_ilp32d_lp64d, implicit-def dead $x1, implicit-def $x2
+    ; CHECK-NEXT: ADJCALLSTACKUP 0, 0, implicit-def dead $x2, implicit $x2
+    ; CHECK-NEXT: $x10 = LD %stack.1, 0 :: (load (s64) from %stack.1)
+    ; CHECK-NEXT: $x1 = LD %stack.0, 0 :: (load (s64) from %stack.0)
+    ; CHECK-NEXT: PseudoRET implicit $x10, implicit $x1, implicit $x8, implicit $x9, implicit $x18, implicit $x19, implicit $x20, implicit $x21, implicit $x22, implicit $x23, implicit $x24, implicit $x25, implicit $x26, implicit $x27
+    %14:gpr = COPY $x27
+    %13:gpr = COPY $x26
+    %12:gpr = COPY $x25
+    %11:gpr = COPY $x24
+    %10:gpr = COPY $x23
+    %9:gpr = COPY $x22
+    %8:gpr = COPY $x21
+    %7:gpr = COPY $x20
+    %6:gpr = COPY $x19
+    %5:gpr = COPY $x18
+    %4:gprc_and_sr07 = COPY $x9
+    %3:gprc_and_sr07 = COPY $x8
+    %2:gpr = COPY $x1
+    %0:gpr = COPY $x10
+    %1:gpr = ADDI %0, 1
+    ADJCALLSTACKDOWN 0, 0, implicit-def dead $x2, implicit $x2
+    PseudoCALL target-flags(riscv-call) @foo, csr_ilp32d_lp64d, implicit-def dead $x1, implicit-def $x2
+    ADJCALLSTACKUP 0, 0, implicit-def dead $x2, implicit $x2
+    $x10 = COPY %1
+    $x1 = COPY %2
+    $x8 = COPY %3
+    $x9 = COPY %4
+    $x18 = COPY %5
+    $x19 = COPY %6
+    $x20 = COPY %7
+    $x21 = COPY %8
+    $x22 = COPY %9
+    $x23 = COPY %10
+    $x24 = COPY %11
+    $x25 = COPY %12
+    $x26 = COPY %13
+    $x27 = COPY %14
+    PseudoRET implicit $x10, implicit $x1, implicit $x8, implicit $x9, implicit $x18, implicit $x19, implicit $x20, implicit $x21, implicit $x22, implicit $x23, implicit $x24, implicit $x25, implicit $x26, implicit $x27
+
+...