update_llc_test_checks.py doesn't report when a common prefix has been lost

[RKSimon]

Bugzilla Link	49189
Version	trunk
OS	Windows NT
CC	@topperc,@gregbedwell,@MaskRay,@LebedevRI,@mtrofin

Extended Description

https://reviews.llvm.org/rGf32dc6bb42fd24517289fd1e3418994daa4fec20

; RUN: llc < %s -mtriple=x86_64-- -mattr=+avx2 | FileCheck %s --check-prefixes=AVX,AVX2
; RUN: llc < %s -mtriple=x86_64-- -mattr=+avx2,+fast-variable-shuffle | FileCheck %s --check-prefixes=AVX,AVX2

update_llc_test_checks.py didn't report that two AVX2 runs had conflicting checks for all their common prefixes, which meant we lost ALL test coverage.

[mtrofin]

FWIW, I don't believe update_llc_test_checks.py ever reported (at least, not consistently) that a given RUN line ends up having no used prefix.

[RKSimon]

FWIW, I don't believe update_llc_test_checks.py ever reported (at least, not
consistently) that a given RUN line ends up having no used prefix.

Prior to D93078, we would warn when we found a mismatch under the last remaining common prefix:

warn('Found conflicting asm under the same prefix: %r!' % (prefix,))

[mtrofin]

Which, as D93078 details, wasn't actually working; in this case, in particular, it wasn't (try sync-ing before that patch and running update_llc_test_checks)

[RKSimon]

@asb does #159321 fix this?

[asb]

Based on the description I believe it should.

[RKSimon]

Confirmed - fixed by #159321 - we're finally getting warnings about missing test checks - thanks @asb!