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The main goal of the model is to avoid *increasing* function size, as
that would eradicate any memory locality benefits from splitting. This
happens when:
- There are too many inputs or outputs to the cold region. Argument
materialization and reloads of outputs have a cost.
- The cold region has too many distinct exit blocks, causing a large
switch to be formed in the caller.
- The code size cost of the split code is less than the cost of a
set-up call.
A secondary goal is to prevent excessive overall binary size growth.
With the cost model in place, I experimented to find a splitting
threshold that works well in practice. To make warm & cold code easily
separable for analysis purposes, I moved split functions to a "cold"
section. I experimented with thresholds between [0, 4] and set the
default to the threshold which minimized geomean __text size.
Experiment data from building LNT+externals for X86 (N = 639 programs,
all sizes in bytes):
| Configuration | __text geom size | __cold geom size | TEXT geom size |
| **-Os** | 1736.3 | 0, n=0 | 10961.6 |
| -Os, thresh=0 | 1740.53 | 124.482, n=134 | 11014 |
| -Os, thresh=1 | 1734.79 | 57.8781, n=90 | 10978.6 |
| -Os, thresh=2 | ** 1733.85 ** | 65.6604, n=61 | 10977.6 |
| -Os, thresh=3 | 1733.85 | 65.3071, n=61 | 10977.6 |
| -Os, thresh=4 | 1735.08 | 67.5156, n=54 | 10965.7 |
| **-Oz** | 1554.4 | 0, n=0 | 10153 |
| -Oz, thresh=2 | ** 1552.2 ** | 65.633, n=61 | 10176 |
| **-O3** | 2563.37 | 0, n=0 | 13105.4 |
| -O3, thresh=2 | ** 2559.49 ** | 71.1072, n=61 | 13162.4 |
Picking thresh=2 reduces the geomean __text section size by 0.14% at
-Os, -Oz, and -O3 and causes ~0.2% growth in the TEXT segment. Note that
TEXT size is page-aligned, whereas section sizes are byte-aligned.
Experiment data from building LNT+externals for ARM64 (N = 558 programs,
all sizes in bytes):
| Configuration | __text geom size | __cold geom size | TEXT geom size |
| **-Os** | 1763.96 | 0, n=0 | 42934.9 |
| -Os, thresh=2 | ** 1760.9 ** | 76.6755, n=61 | 42934.9 |
Picking thresh=2 reduces the geomean __text section size by 0.17% at
-Os and causes no growth in the TEXT segment.
Measurements were done with D57082 (r352080) applied.
Differential Revision: https://reviews.llvm.org/D57125
llvm-svn: 352228
58 lines
1.1 KiB
LLVM
58 lines
1.1 KiB
LLVM
; RUN: opt -S -hotcoldsplit -hotcoldsplit-threshold=-1 < %s 2>&1 | FileCheck %s
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; CHECK-LABEL: define {{.*}}@fun
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; CHECK: call {{.*}}@fun.cold.2(
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; CHECK-NEXT: ret void
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; CHECK: call {{.*}}@fun.cold.1(
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; CHECK-NEXT: ret void
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define void @fun() {
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entry:
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br i1 undef, label %A.then, label %A.else
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A.else:
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br label %A.then4
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A.then4:
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br i1 undef, label %A.then5, label %A.end
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A.then5:
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br label %A.cleanup
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A.end:
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br label %A.cleanup
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A.cleanup:
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%A.cleanup.dest.slot.0 = phi i32 [ 1, %A.then5 ], [ 0, %A.end ]
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unreachable
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A.then:
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br i1 undef, label %B.then, label %B.else
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B.then:
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ret void
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B.else:
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br label %B.then4
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B.then4:
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br i1 undef, label %B.then5, label %B.end
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B.then5:
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br label %B.cleanup
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B.end:
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br label %B.cleanup
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B.cleanup:
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%B.cleanup.dest.slot.0 = phi i32 [ 1, %B.then5 ], [ 0, %B.end ]
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unreachable
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}
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; CHECK-LABEL: define {{.*}}@fun.cold.1(
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; CHECK: %B.cleanup.dest.slot.0 = phi i32 [ 1, %B.then5 ], [ 0, %B.end ]
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; CHECK-NEXT: unreachable
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; CHECK-LABEL: define {{.*}}@fun.cold.2(
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; CHECK: %A.cleanup.dest.slot.0 = phi i32 [ 1, %A.then5 ], [ 0, %A.end ]
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; CHECK-NEXT: unreachable
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