Instructions to reproduce:

• git clone git@github.com:servo/html5ever.git
• cd html5ever/markup5ever
• CARGO_INCREMENTAL=0 RUSTFLAGS="-Zprofile -Ccodegen-units=1 -Cinline-threshold=0" cargo check
• The build script of markup5ever segfaults.

The actual function with the bug is an instance std::panicking::try in the proc_macro2 crate.

The cause seems to be an LLVM bug.

full gist

This LLVM IR is generated:

catch.i:                                          ; preds = %.noexc
  %120 = phi i64* [ getelementptr inbounds ([24 x i64], [24 x i64]* @__llvm_gcov_ctr.27, i64 0, i64 11), %.noexc ], !dbg !2861
  %121 = landingpad { i8*, i32 }
          catch i8* null, !dbg !2861
  %122 = load i64, i64* %120, !dbg !2861
  %123 = add i64 %122, 1, !dbg !2861
  store i64 %123, i64* %120, !dbg !2861```

Notice how the phi is inserted before the landingpad instruction. This causes the following asm to be generated:

.LBB27_14: // This is never executed
	.loc	27 0 15 is_stmt 0
	movq	160(%rsp), %rcx
	movl	$1, %esi
.Ltmp379: // Landing pad points to here!!!
	leaq	__llvm_gcov_ctr.27(%rip), %rdi
	addq	$120, %rdi
	.loc	27 274 15
	movq	(%rcx), %r8
	addq	$1, %r8
	movq	%r8, (%rcx)

So basically the initialization of %rcx is getting skipped by the incorrect landing pad, which in turn causes the crash.

Edit by @Amanieu, original bug report follows.

Just updated nightly on my CI machine

nightly-aarch64-unknown-linux-gnu updated - rustc 1.44.0-nightly (f509b26 2020-03-18) (from rustc 1.43.0-nightly (c20d7ee 2020-03-11))

and found out that tests stopped compiling few of dependencies like cssparser or string_cache or html5ever.

It probably happens because of my RUSTFLAGS

CARGO_INCREMENTAL=0;
RUSTFLAGS="-Zprofile -Ccodegen-units=1 -Cinline-threshold=0 -Coverflow-checks=off -Zno-landing-pads";

I created repository with reproduction
https://github.com/Lesiuk/rust-nightly-issue-reproduction

Bissect found that this PR introduced this issue #67502

searched toolchains c20d7ee through 3c6f982
regression in be055d9

Log from test run

Compiling html5ever v0.25.1
error: failed to run custom build command for html5ever v0.25.1

Caused by:
process didn't exit successfully: /Users/XXXXXX/CLionProjects/issue/target/debug/build/html5ever-1a979961379450d7/build-script-build (signal: 6, SIGABRT: process abort signal)
--- stdout
cargo:rerun-if-changed=/Users/XXXXXX/.cargo/registry/src/github.com-1ecc6299db9ec823/html5ever-0.25.1/src/tree_builder/rules.rs

--- stderr
fatal runtime error: failed to initiate panic, error 5

warning: build failed, waiting for other jobs to finish...
error: build failed