tree 4d3055735a3b7ef97f6311e70b99394fc87dcdf1
parent 00f59269287658489eda6279e52ae058dc686c96
author Roman Lebedev <lebedev.ri@gmail.com> 1547587878 +0000
committer Roman Lebedev <lebedev.ri@gmail.com> 1547587878 +0000

X86DAGToDAGISel::matchBitExtract() with truncation (PR36419)

Summary:
Previously in D54095 i have added support for extraction of `lshr` from `X` if we are to produce `BEXTR`.
That was good, but the fix was partial, there was still [[ https://bugs.llvm.org/show_bug.cgi?id=36419 | PR36419 ]].

That pattern can also appear, roughly, when you have a large (64-bit) storage, and the consume bits from it.
It will not be unexpected if you will be doing further computations in 32-bit width.
And then the current code breaks, as the tests show.

The basic idea/pattern here is following:
1. We have `i64` input
2. We perform `i64` right-shift on it.
3. We `trunc`ate that shifted value
4. We do all further work (masking) in `i32`

Since we see `trunc`ation and not `lshr`, we give up, and stop trying to extract that right-shift.
BUT. The mask is `i32`, therefore we can extend both of the operands of the masking (`and`) to `i64`
and truncate the result after masking: https://rise4fun.com/Alive/K4B
```
Name: @bextr64_32_b1 -> @bextr64_32_b0
  %shiftedval = lshr i64 %val, %numskipbits
  %truncshiftedval = trunc i64 %shiftedval to i32
  %widenumlowbits1 = zext i8 %numlowbits to i32
  %notmask1 = shl nsw i32 -1, %widenumlowbits1
  %mask1 = xor i32 %notmask1, -1
  %res = and i32 %truncshiftedval, %mask1
=>
  %shiftedval = lshr i64 %val, %numskipbits
  %widenumlowbits = zext i8 %numlowbits to i64
  %notmask = shl nsw i64 -1, %widenumlowbits
  %mask = xor i64 %notmask, -1
  %wideres = and i64 %shiftedval, %mask
  %res = trunc i64 %wideres to i32
```

Thus, we are again able to extract that `lshr` into `BEXTR`'s control.

Now, the perf (via `llvm-exegesis`) of the snippet suggests that it is not a good idea:
```
$ cat /tmp/old.s
# bextr64_32_b1
# LLVM-EXEGESIS-LIVEIN RSI
# LLVM-EXEGESIS-LIVEIN EDX
# LLVM-EXEGESIS-LIVEIN RDI
movq %rsi, %rcx
shrq %cl, %rdi
shll $8, %edx
bextrl %edx, %edi, %eax
$ cat /tmp/old.s | ./bin/llvm-exegesis -mode=latency -snippets-file=-
Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-1e0082.o
---
mode:            latency
key:
  instructions:
    - 'MOV64rr RCX RSI'
    - 'SHR64rCL RDI RDI'
    - 'SHL32ri EDX EDX i_0x8'
    - 'BEXTR32rr EAX EDI EDX'
  config:          ''
  register_initial_values: []
cpu_name:        bdver2
llvm_triple:     x86_64-unknown-linux-gnu
num_repetitions: 10000
measurements:
  - { key: latency, value: 0.6638, per_snippet_value: 2.6552 }
error:           ''
info:            ''
assembled_snippet: 4889F148D3EFC1E208C4E268F7C74889F148D3EFC1E208C4E268F7C74889F148D3EFC1E208C4E268F7C74889F148D3EFC1E208C4E268F7C7C3
...
$ cat /tmp/old.s | ./bin/llvm-exegesis -mode=uops -snippets-file=-
Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-43e346.o
---
mode:            uops
key:
  instructions:
    - 'MOV64rr RCX RSI'
    - 'SHR64rCL RDI RDI'
    - 'SHL32ri EDX EDX i_0x8'
    - 'BEXTR32rr EAX EDI EDX'
  config:          ''
  register_initial_values: []
cpu_name:        bdver2
llvm_triple:     x86_64-unknown-linux-gnu
num_repetitions: 10000
measurements:
  - { key: PdFPU0, value: 0, per_snippet_value: 0 }
  - { key: PdFPU1, value: 0, per_snippet_value: 0 }
  - { key: PdFPU2, value: 0, per_snippet_value: 0 }
  - { key: PdFPU3, value: 0, per_snippet_value: 0 }
  - { key: NumMicroOps, value: 1.2571, per_snippet_value: 5.0284 }
error:           ''
info:            ''
assembled_snippet: 4889F148D3EFC1E208C4E268F7C74889F148D3EFC1E208C4E268F7C74889F148D3EFC1E208C4E268F7C74889F148D3EFC1E208C4E268F7C7C3
...
```
vs
```
$ cat /tmp/new.s
# bextr64_32_b1
# LLVM-EXEGESIS-LIVEIN RDX
# LLVM-EXEGESIS-LIVEIN SIL
# LLVM-EXEGESIS-LIVEIN RDI
shlq $8, %rdx
movzbl %sil, %eax
orq %rdx, %rax
bextrq %rax, %rdi, %rax
$ cat /tmp/new.s | ./bin/llvm-exegesis -mode=latency -snippets-file=-
Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-8944f1.o
---
mode:            latency
key:
  instructions:
    - 'SHL64ri RDX RDX i_0x8'
    - 'MOVZX32rr8 EAX SIL'
    - 'OR64rr RAX RAX RDX'
    - 'BEXTR64rr RAX RDI RAX'
  config:          ''
  register_initial_values: []
cpu_name:        bdver2
llvm_triple:     x86_64-unknown-linux-gnu
num_repetitions: 10000
measurements:
  - { key: latency, value: 0.7454, per_snippet_value: 2.9816 }
error:           ''
info:            ''
assembled_snippet: 48C1E208400FB6C64809D0C4E2F8F7C748C1E208400FB6C64809D0C4E2F8F7C748C1E208400FB6C64809D0C4E2F8F7C748C1E208400FB6C64809D0C4E2F8F7C7C3
...
$ cat /tmp/new.s | ./bin/llvm-exegesis -mode=uops -snippets-file=-
Check generated assembly with: /usr/bin/objdump -d /tmp/snippet-da403c.o
---
mode:            uops
key:
  instructions:
    - 'SHL64ri RDX RDX i_0x8'
    - 'MOVZX32rr8 EAX SIL'
    - 'OR64rr RAX RAX RDX'
    - 'BEXTR64rr RAX RDI RAX'
  config:          ''
  register_initial_values: []
cpu_name:        bdver2
llvm_triple:     x86_64-unknown-linux-gnu
num_repetitions: 10000
measurements:
  - { key: PdFPU0, value: 0, per_snippet_value: 0 }
  - { key: PdFPU1, value: 0, per_snippet_value: 0 }
  - { key: PdFPU2, value: 0, per_snippet_value: 0 }
  - { key: PdFPU3, value: 0, per_snippet_value: 0 }
  - { key: NumMicroOps, value: 1.2571, per_snippet_value: 5.0284 }
error:           ''
info:            ''
assembled_snippet: 48C1E208400FB6C64809D0C4E2F8F7C748C1E208400FB6C64809D0C4E2F8F7C748C1E208400FB6C64809D0C4E2F8F7C748C1E208400FB6C64809D0C4E2F8F7C7C3
...
```
^ latency increased (worse).

Except //maybe// not really.
Like with all synthetic benchmarks, they //may// be misleading.

Let's take a look on some actual real-world hotpath.
In this case it's 'my' [[ https://github.com/darktable-org/rawspeed | RawSpeed ]]'s `BitStream<>::peekBitsNoFill()`, in [[ https://github.com/darktable-org/rawspeed/blob/e3316dc85127c2c29baa40f998f198a7b278bf36/src/librawspeed/decompressors/VC5Decompressor.cpp#L814 | GoPro VC5 decompressor ]]:
```
raw.pixls.us-unique/GoPro/HERO6 Black$ /usr/src/googlebenchmark/tools/compare.py -a benchmarks ~/rawspeed/build-clangs1-{old,new}/src/utilities/rsbench/rsbench --benchmark_counters_tabular=true --benchmark_min_time=0.00000001 --benchmark_repetitions=128 GOPR9172.GPR
RUNNING: /home/lebedevri/rawspeed/build-clangs1-old/src/utilities/rsbench/rsbench --benchmark_counters_tabular=true --benchmark_min_time=0.00000001 --benchmark_repetitions=128 GOPR9172.GPR --benchmark_display_aggregates_only=true --benchmark_out=/tmp/tmplwbKEM
2018-12-22 21:23:03
Running /home/lebedevri/rawspeed/build-clangs1-old/src/utilities/rsbench/rsbench
Run on (8 X 4012.81 MHz CPU s)
CPU Caches:
  L1 Data 16K (x8)
  L1 Instruction 64K (x4)
  L2 Unified 2048K (x4)
  L3 Unified 8192K (x1)
Load Average: 3.41, 2.41, 2.03
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Benchmark                                        Time           CPU Iterations  CPUTime,s CPUTime/WallTime     Pixels Pixels/CPUTime Pixels/WallTime Raws/CPUTime Raws/WallTime WallTime,s
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
GOPR9172.GPR/threads:8/real_time_mean           40 ms         40 ms        128   0.322244          7.96974        12M       37.4457M        298.534M      3.12047       24.8778   0.040465
GOPR9172.GPR/threads:8/real_time_median         39 ms         39 ms        128   0.312606          7.99155        12M        38.387M        306.788M      3.19891       25.5656   0.039115
GOPR9172.GPR/threads:8/real_time_stddev          4 ms          3 ms        128  0.0271557         0.130575          0        2.4941M        21.3909M     0.207842       1.78257   3.81081m
RUNNING: /home/lebedevri/rawspeed/build-clangs1-new/src/utilities/rsbench/rsbench --benchmark_counters_tabular=true --benchmark_min_time=0.00000001 --benchmark_repetitions=128 GOPR9172.GPR --benchmark_display_aggregates_only=true --benchmark_out=/tmp/tmpWAkan9
2018-12-22 21:23:08
Running /home/lebedevri/rawspeed/build-clangs1-new/src/utilities/rsbench/rsbench
Run on (8 X 4013.1 MHz CPU s)
CPU Caches:
  L1 Data 16K (x8)
  L1 Instruction 64K (x4)
  L2 Unified 2048K (x4)
  L3 Unified 8192K (x1)
Load Average: 3.78, 2.50, 2.06
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Benchmark                                        Time           CPU Iterations  CPUTime,s CPUTime/WallTime     Pixels Pixels/CPUTime Pixels/WallTime Raws/CPUTime Raws/WallTime WallTime,s
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
GOPR9172.GPR/threads:8/real_time_mean           39 ms         39 ms        128   0.311533          7.97323        12M       38.6828M        308.471M      3.22356        25.706  0.0390928
GOPR9172.GPR/threads:8/real_time_median         38 ms         38 ms        128   0.304231          7.99005        12M       39.4437M        315.527M      3.28698        26.294  0.0380316
GOPR9172.GPR/threads:8/real_time_stddev          3 ms          3 ms        128  0.0229149         0.133814          0       2.26225M        19.1421M     0.188521       1.59517   3.13671m
Comparing /home/lebedevri/rawspeed/build-clangs1-old/src/utilities/rsbench/rsbench to /home/lebedevri/rawspeed/build-clangs1-new/src/utilities/rsbench/rsbench
Benchmark                                                 Time             CPU      Time Old      Time New       CPU Old       CPU New
--------------------------------------------------------------------------------------------------------------------------------------
GOPR9172.GPR/threads:8/real_time_pvalue                 0.0000          0.0000      U Test, Repetitions: 128 vs 128
GOPR9172.GPR/threads:8/real_time_mean                  -0.0339         -0.0316            40            39            40            39
GOPR9172.GPR/threads:8/real_time_median                -0.0277         -0.0274            39            38            39            38
GOPR9172.GPR/threads:8/real_time_stddev                -0.1769         -0.1267             4             3             3             3
```
I.e. this results in //roughly// -3% improvements in perf.

While this will help [[ https://bugs.llvm.org/show_bug.cgi?id=36419 | PR36419 ]], it won't address it fully.

Reviewers: RKSimon, craig.topper, andreadb, spatel

Reviewed By: craig.topper

Subscribers: courbet, llvm-commits

Differential Revision: https://reviews.llvm.org/D56052

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351253 91177308-0d34-0410-b5e6-96231b3b80d8