tree 0deeac5b1230741ddb73eddafbae821cc80052fc
parent 43eae8d7784a60d7c077c8638b4f512bfc666fd0
author Thurston Dang <thurston@google.com> 1750802984 -0700
committer Copybara-Service <copybara-worker@google.com> 1750803064 -0700

[msan] Automatically print shadow for failing outlined checks (#145107)

A commonly used aid for debugging MSan reports is `__msan_print_shadow()`, which requires manual app code annotations (typically of the variable in the UUM report or nearby). This is in contrast to ASan, which automatically prints out the shadow map when a check fails.

This patch changes MSan to print the shadow that failed an outlined check (checks are outlined per function after the `-msan-instrumentation-with-call-threshold` is exceeded) if verbosity >= 1. Note that we do not print out the shadow map of "neighboring" variables because this is technically infeasible; see "Caveat" below.

This patch can be easier to use than `__msan_print_shadow()` because this does not require manual app code annotations. Additionally, due to optimizations, `__msan_print_shadow()` calls can sometimes spuriously affect whether a variable is initialized.

As a side effect, this patch also enables outlined checks for arbitrary-sized shadows (vs. the current hardcoded handlers for {1,2,4,8}-byte shadows).

Caveat: the shadow does not necessarily correspond to an individual user variable, because MSan instrumentation may combine and/or truncate multiple shadows prior to emitting a check that the mangled shadow is zero (e.g., `convertShadowToScalar()`, `handleSSEVectorConvertIntrinsic()`, `materializeInstructionChecks()`). OTOH it is arguably a strength that this feature emit the shadow that directly matters for the MSan check, but which cannot be obtained using the MSan API.
GitOrigin-RevId: 1b71ea411a9d36705663b1724ececbdfec7cc98c