Compilation crash targetting wasm with -O2 and reference types

[hoodmane]

Tested against:

Ubuntu clang version 17.0.3 (++20231017073226+888437e1b600-1exp120231017073338.54)
clang version 18.0.0 (https://github.com/llvm/llvm-project d1e685d)

cc @sbc100 @pmatos

testcase.c

__externref_t foo(void);
void bar(__externref_t);

void
test(int flag, __externref_t ref1, __externref_t ref2)
{
  if (flag) {
    ref1 = foo();
    ref2 = foo();
  }
  bar(ref1);
  bar(ref2);
}

Compilation command:

clang -c testcase.c -O2   -mreference-types -target wasm32-unknown-unknown

The files it asked me to upload:

testcase-ccc7e1.c

# 1 "<built-in>"
# 1 "testcase.c"
__externref_t foo(void);
void bar(__externref_t);

void
test(int flag, __externref_t ref1, __externref_t ref2)
{
  if (flag) {
    ref1 = foo();
    ref2 = foo();
  }
  bar(ref1);
  bar(ref2);
}

testcase-ccc7e1.sh

# Crash reproducer for Ubuntu clang version 17.0.3 (++20231017073226+888437e1b600-1~exp1~20231017073338.54)
# Driver args: "-c" "testcase.c" "-O2" "-mreference-types" "-target" "wasm32-unknown-unknown"
# Original command:  "/usr/lib/llvm-17/bin/clang" "-cc1" "-triple" "wasm32-unknown-unknown" "-emit-obj" "-disable-free" "-clear-ast-before-backend" "-disable-llvm-verifier" "-discard-value-names" "-main-file-name" "testcase.c" "-mrelocation-model" "static" "-mframe-pointer=none" "-ffp-contract=on" "-fno-rounding-math" "-mconstructor-aliases" "-target-cpu" "generic" "-target-feature" "+reference-types" "-fvisibility=hidden" "-debugger-tuning=gdb" "-fcoverage-compilation-dir=/home/hood/Documents/programming/pyodide" "-resource-dir" "/usr/lib/llvm-17/lib/clang/17" "-internal-isystem" "/usr/lib/llvm-17/lib/clang/17/include" "-O2" "-fdebug-compilation-dir=/home/hood/Documents/programming/pyodide" "-ferror-limit" "19" "-fgnuc-version=4.2.1" "-fcolor-diagnostics" "-vectorize-loops" "-vectorize-slp" "-o" "testcase.o" "-x" "c" "testcase.c"
 "/usr/lib/llvm-17/bin/clang" "-cc1" "-triple" "wasm32-unknown-unknown" "-emit-obj" "-disable-free" "-clear-ast-before-backend" "-disable-llvm-verifier" "-discard-value-names" "-main-file-name" "testcase.c" "-mrelocation-model" "static" "-mframe-pointer=none" "-ffp-contract=on" "-fno-rounding-math" "-mconstructor-aliases" "-target-cpu" "generic" "-target-feature" "+reference-types" "-fvisibility=hidden" "-debugger-tuning=gdb" "-fcoverage-compilation-dir=/home/hood/Documents/programming/pyodide" "-O2" "-fdebug-compilation-dir=/home/hood/Documents/programming/pyodide" "-ferror-limit" "19" "-fgnuc-version=4.2.1" "-fcolor-diagnostics" "-vectorize-loops" "-vectorize-slp" "-x" "c" "testcase-ccc7e1.c"

[llvmbot]

@llvm/issue-subscribers-backend-webassembly

Author: Hood Chatham (hoodmane)

Tested against:

Ubuntu clang version 17.0.3 (++20231017073226+888437e1b600-1exp120231017073338.54)
clang version 18.0.0 (https://github.com/llvm/llvm-project d1e685d)

cc @sbc100 @pmatos

testcase.c

__externref_t foo(void);
void bar(__externref_t);

void
test(int flag, __externref_t ref1, __externref_t ref2)
{
  if (flag) {
    ref1 = foo();
    ref2 = foo();
  }
  bar(ref1);
  bar(ref2);
}

Compilation command:

clang -c testcase.c -O2   -mreference-types -target wasm32-unknown-unknown

The files it asked me to upload:

testcase-ccc7e1.c

# 1 "<built-in>"
# 1 "testcase.c"
__externref_t foo(void);
void bar(__externref_t);

void
test(int flag, __externref_t ref1, __externref_t ref2)
{
  if (flag) {
    ref1 = foo();
    ref2 = foo();
  }
  bar(ref1);
  bar(ref2);
}

testcase-ccc7e1.sh

# Crash reproducer for Ubuntu clang version 17.0.3 (++20231017073226+888437e1b600-1~exp1~20231017073338.54)
# Driver args: "-c" "testcase.c" "-O2" "-mreference-types" "-target" "wasm32-unknown-unknown"
# Original command:  "/usr/lib/llvm-17/bin/clang" "-cc1" "-triple" "wasm32-unknown-unknown" "-emit-obj" "-disable-free" "-clear-ast-before-backend" "-disable-llvm-verifier" "-discard-value-names" "-main-file-name" "testcase.c" "-mrelocation-model" "static" "-mframe-pointer=none" "-ffp-contract=on" "-fno-rounding-math" "-mconstructor-aliases" "-target-cpu" "generic" "-target-feature" "+reference-types" "-fvisibility=hidden" "-debugger-tuning=gdb" "-fcoverage-compilation-dir=/home/hood/Documents/programming/pyodide" "-resource-dir" "/usr/lib/llvm-17/lib/clang/17" "-internal-isystem" "/usr/lib/llvm-17/lib/clang/17/include" "-O2" "-fdebug-compilation-dir=/home/hood/Documents/programming/pyodide" "-ferror-limit" "19" "-fgnuc-version=4.2.1" "-fcolor-diagnostics" "-vectorize-loops" "-vectorize-slp" "-o" "testcase.o" "-x" "c" "testcase.c"
 "/usr/lib/llvm-17/bin/clang" "-cc1" "-triple" "wasm32-unknown-unknown" "-emit-obj" "-disable-free" "-clear-ast-before-backend" "-disable-llvm-verifier" "-discard-value-names" "-main-file-name" "testcase.c" "-mrelocation-model" "static" "-mframe-pointer=none" "-ffp-contract=on" "-fno-rounding-math" "-mconstructor-aliases" "-target-cpu" "generic" "-target-feature" "+reference-types" "-fvisibility=hidden" "-debugger-tuning=gdb" "-fcoverage-compilation-dir=/home/hood/Documents/programming/pyodide" "-O2" "-fdebug-compilation-dir=/home/hood/Documents/programming/pyodide" "-ferror-limit" "19" "-fgnuc-version=4.2.1" "-fcolor-diagnostics" "-vectorize-loops" "-vectorize-slp" "-x" "c" "testcase-ccc7e1.c"

[pmatos]

@hoodmane thanks for the report, I will take a look.

[pmatos]

I can reproduce, will work on a fix.

[pmatos]

The SLP vectorizer is trying to create vector of externref elements (illegal!!!) to calculate their cost. It's one of those places where we need to add an exception and say that values of wasm reference type cannot be valid vector elements.

[pmatos]

Added a PR with a fix but this cannot go upstream at the moment. We need to work on a better fix for this. This patch is just to try to gather ideas on the best way to move forward.

[pmatos]

@hoodmane this is one of those simple bugs to understand but the fix might be more involved. If you need a workaround for WebAssembly at the moment you can apply the patch in #71069 locally.