clang-20 crashed with optnone attribute at -O1 and above. error in backend: Cannot select: intrinsic %llvm.expect.with.probability.

[iamanonymouscs]

clang-20 crashed with optnone attribute at -O1 and above.

Compiler explorer: https://godbolt.org/z/4dP5xGxnx

$cat mutant.c
int a;
void __attribute__((optnone)) b() {
  int c = __builtin_expect_with_probability(a, 0, 0.8);
}

Also crashed on clang-19.

$clang-19 -O1 mutant.c
fatal error: error in backend: Cannot select: intrinsic %llvm.expect.with.probability
clang: error: clang frontend command failed with exit code 70 (use -v to see invocation)
Ubuntu clang version 19.0.0 (++20240722031324+65825cd5431c-1~exp1~20240722151445.1819)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-19/bin
clang: note: diagnostic msg: 
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang: note: diagnostic msg: /tmp/mutant-b18d19.c
clang: note: diagnostic msg: /tmp/mutant-b18d19.sh
clang: note: diagnostic msg: 

********************

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Anonymous (iamanonymouscs)

clang-20 crashed with ```optnone``` attribute at ```-O1``` and above.

Compiler explorer: https://godbolt.org/z/4dP5xGxnx

$cat mutant.c
int a;
void __attribute__((optnone)) b() {
  int c = __builtin_expect_with_probability(a, 0, 0.8);
}

Also crashed on clang-19.

$clang-19 -O1 mutant.c
fatal error: error in backend: Cannot select: intrinsic %llvm.expect.with.probability
clang: error: clang frontend command failed with exit code 70 (use -v to see invocation)
Ubuntu clang version 19.0.0 (++20240722031324+65825cd5431c-1~exp1~20240722151445.1819)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-19/bin
clang: note: diagnostic msg: 
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang: note: diagnostic msg: /tmp/mutant-b18d19.c
clang: note: diagnostic msg: /tmp/mutant-b18d19.sh
clang: note: diagnostic msg: 

********************

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Anonymous (iamanonymouscs)

clang-20 crashed with ```optnone``` attribute at ```-O1``` and above.

Compiler explorer: https://godbolt.org/z/4dP5xGxnx

$cat mutant.c
int a;
void __attribute__((optnone)) b() {
  int c = __builtin_expect_with_probability(a, 0, 0.8);
}

Also crashed on clang-19.

$clang-19 -O1 mutant.c
fatal error: error in backend: Cannot select: intrinsic %llvm.expect.with.probability
clang: error: clang frontend command failed with exit code 70 (use -v to see invocation)
Ubuntu clang version 19.0.0 (++20240722031324+65825cd5431c-1~exp1~20240722151445.1819)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-19/bin
clang: note: diagnostic msg: 
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang: note: diagnostic msg: /tmp/mutant-b18d19.c
clang: note: diagnostic msg: /tmp/mutant-b18d19.sh
clang: note: diagnostic msg: 

********************

[phoebewang]

I think this is Clang codegen problem. According to https://github.com/llvm/llvm-project/blob/main/clang/lib/CodeGen/CGBuiltin.cpp#L3522, we should not generate llvm.expect.with.probability for optnone as well.

[antangelo]

That link seems to have drifted, but I presume it means to link to this:

llvm-project/clang/lib/CodeGen/CGBuiltin.cpp

Lines 3573 to 3577 in 93a4244

	// Don't generate llvm.expect.with.probability on -O0 as the backend
	// won't use it for anything.
	// Note, we still IRGen ExpectedValue because it could have side-effects.
	if (CGM.getCodeGenOpts().OptimizationLevel == 0)
	return RValue::get(ArgValue);

Curiously, if you replace __builtin_expect_with_probability with __builtin_expect, there is no assertion failure in the backend (despite the optimization check on the Clang side being exactly the same). See https://godbolt.org/z/abeh8Wba5

It seems like SelectionDAGBuilder, FastISel, and IntrinsicLowering each have similar handling that removes the llvm.expect if it has not already been transformed out, but no such handling exists for llvm.expect.with.probability. I think it should be handled here for consistency, but it probably doesn't hurt to check for optnone on the clang side too.