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[RegAllocEvictAdvisor] Add minimum weight ratio heuristic. #98109

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[RegAllocEvictAdvisor] Add minimum weight ratio heuristic. #98109

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[RegAllocEvictAdvisor][NFC] Add minimum weight ratio heuristic.
mgudim Jul 9, 2024
3c5578a
Trigger CI.
mgudim Jul 11, 2024
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18 changes: 16 additions & 2 deletions llvm/lib/CodeGen/RegAllocEvictionAdvisor.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -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,
Expand Down Expand Up @@ -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 &&
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The CanSplit doesn't make much sense to me here.
Could you explain why this is desirable to check for this here?

(WeightRatio < MinWeightRatioNeededToEvictHint))
return false;
LLVM_DEBUG(dbgs() << "should evict: " << B << " w= " << BWeight << '\n');
return true;
}
return false;
Expand Down
76 changes: 76 additions & 0 deletions llvm/test/CodeGen/RISCV/min-weight-ratio.mir
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@@ -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
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To go back to what I was saying wrt 7.5. I'm not questioning that this gives you the best outcome in what you measured, what I was asking is why it is.
Essentially what is the explanation that makes 7.5 a good number aside from "it fixes my problem".


# 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

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