Conversation
zend_cpu_startup() masked ZEND_CPU_FEATURE_AVX out of EDX when XCR0 lacks the SSE or AVX state bits, but AVX is CPUID.1:ECX bit 28 and EDX bit 28 is HTT. zend_cpu_supports(ZEND_CPU_FEATURE_AVX) kept reporting AVX, so builds without __builtin_cpu_supports(), such as MSVC, let the JIT emit VEX-encoded instructions that fault on such hosts.
ndossche
approved these changes
Sep 28, 2026
ndossche
left a comment
Member
There was a problem hiding this comment.
Wow this is surprising.
Good find
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
zend_cpu_startup() cleared the AVX bit from EDX, where bit 28 is HTT, instead of ECX, so AVX stayed reported when XCR0 has AVX state disabled. Builds without __builtin_cpu_supports(), such as MSVC, then let the JIT emit VEX-encoded instructions that fault on such hosts. The mask dates from 4e30ab3. There is no test, since it needs a host with AVX state disabled in XCR0.