AVX-512: Unable to copy EFLAGS physical register!

[llvmbot]

Bugzilla Link	38803
Resolution	FIXED
Resolved on	Oct 19, 2018 12:14
Version	7.0
OS	Linux
Blocks	#38454
Attachments	tab.cpp
Reporter	LLVM Bugzilla Contributor
CC	@chandlerc,@topperc,@francisvm,@RKSimon,@tstellar
Fixed by commit(s)	r343443 r343428 r344804 r344805

Extended Description

We've got a bug report from one of our users attempting to build Chromium using clang. The build is failing for him with the following error message:

fatal error: error in backend: Unable to copy EFLAGS physical register!
x86_64-pc-linux-gnu-clang++: error: clang frontend command failed with exit code 70 (use -v to see invocation)

I'm attaching a minimal test case prepared by Sergei Trofimovich:

// $ clang++ -march=skylake-avx512 -O2 tab.cpp
// fatal error: error in backend: Unable to copy EFLAGS physical register!
bool a();
int b, c;
float d;

float fn2() {
int e = a();
double f = e ? 0.75f : 0;
if (c)
d = e ? 0 : b;
return f;
}

I was able to reproduce the issue with today's LLVM+Clang trunk.

[topperc]

The error message is a red herring. It's actually hitting an llvm_unreachable if you use a debug build.

Cannot emit physreg copy instruction
UNREACHABLE executed at lib/Target/X86/X86InstrInfo.cpp:3122!

[topperc]

Fixed in r343443

[RKSimon]

Merge into 7.0.1?

[topperc]

Reopening and marking block 7.0.1

[topperc]

We should probably also take r343428 into 7.0.1 so we stop getting misleading bug titles.

[tstellar]

Merged: r344804 r344805

[tstellar]

mentioned in issue #38454