[Bug middle-end/122455] New: [16 regression] Regression in Snappy tests due to incorrect speculation of devirtualization target

[soumyaa at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122455

            Bug ID: 122455
           Summary: [16 regression] Regression in Snappy tests due to
                    incorrect speculation of devirtualization target
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: soumyaa at gcc dot gnu.org
                CC: jh at suse dot cz
  Target Milestone: ---

Hi,

The following Snappy tests:

BM_UValidate/2/1
BM_UValidate/2/2
BM_UValidate/3/1
BM_UValidate/3/2

have observed a ~10% regression after commit 9ee937b2: Add --param
max-devirt-targets [https://gcc.gnu.org/cgit/gcc/commit/?id=9ee937b2]

---

Initial impressions:

The function SnappyDecompressor::RefillTag() calls the functions Peek() and
Skip which are implemented via 2 classes:

- ByteArraySource
- SnappyIOVecReader

This patch seems to pick SnappyIOVecReader as the first speculative target, as
is seen here:

        ldr     x2, [x2, :got_lo12:_ZN6snappy17SnappyIOVecReader4SkipEm]
        ldr     x9, [x0]
        ldr     x3, [x9, 32]
        cmp     x3, x2   # Compare vtable
        beq     .L885
        blr     x3

But, checking with the profile data for this function confirms that we only
call the ByteArraySource implementation:

_ZN6snappy18SnappyDecompressor9RefillTagEv total:1415550 head:81082
0: 81082
1: 81082
2: 81082
4: 80174 _ZN6snappy15ByteArraySource4SkipEm:80988
5: 80370
6: 80682 _ZN6snappy15ByteArraySource4PeekEPm:80489

Depending on the specific test, we might end up calling a different
implementation and the slowdown is consistent across all tests that use the
ByteArraySource implementation.

---

Steps to reproduce:


# Clone
git clone https://github.com/google/snappy.git
cd snappy
git submodule update --init

# Build
mkdir builddir
cd builddir
CC=/path/to/gcc-install-dir/bin/gcc CXX=/path/to/gcc-install-dir/bin/g++
OPT="<compile flags>" SRC=$(pwd)
LD_LIBRARY_PATH=/path/to/gcc-install-dir/lib64:$LD_LIBRARY_PATH
cmake .. -DCMAKE_C_COMPILER=$CC -DCMAKE_CXX_COMPILER=$CXX
-DCMAKE_C_FLAGS="$OPT" -DCMAKE_CXX_FLAGS="$OPT" -DCMAKE_BUILD_TYPE="Release"
${SRC}
make -j $(nproc)

# Run
cp -r ../testdata/ .
./snappy_benchmark --benchmark_filter="BM_UValidate/2/1"
--benchmark_min_warmup_time=2 --benchmark_time_unit=us;