compile/internal/pgo: show informative error message for missing Function.start_line

[lizthegrey]

What version of Go are you using (go version)?

$ go version
go version go1.20rc2 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What did you do?

Pass a default.pgo merged profile that contained some pprof data from 1.19 rather than 1.20rcX.

What did you expect to see?

An informative error message telling me that the profile passed in needed to be entirely from 1.20rcX or newer, specifically containing the patch that enabled start_line passing

What did you see instead?

# vendor/golang.org/x/crypto/cryptobyte/asn1
compile: PGO profile missing Function.start_line data
# sync/atomic
compile: PGO profile missing Function.start_line data
...

[prattmic]

Thanks, this should indeed be more informative.

[gopherbot]

Change https://go.dev/cl/461195 mentions this issue: cmd/compile/internal/pgo: add hint to missing start_line error