ansi-c: support -fms-extensions anonymous tagged struct/union members#9050
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ansi-c: support -fms-extensions anonymous tagged struct/union members#9050tautschnig wants to merge 1 commit into
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Pull request overview
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Adds support for MS-style extensions needed to correctly typecheck and lay out tagged anonymous struct/union members (e.g., Linux kernel patterns) when compiling in GCC/Clang mode with -fms-extensions.
Changes:
- Introduce a new
ansi_c.ms_extensionsconfig flag (defaultfalse) and reset it inconfigt::set. - Enable the flag when
-fms-extensionsis passed throughgoto-gcc. - Extend compound member typechecking to accept tagged anonymous struct/union members only when
ms_extensionsis enabled, and add a regression test validating layout + member injection.
Reviewed changes
Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.
Show a summary per file
| File | Description |
|---|---|
| src/util/config.h | Adds ansi_c.ms_extensions configuration flag. |
| src/util/config.cpp | Initializes/resets ansi_c.ms_extensions during configt::set. |
| src/goto-cc/gcc_mode.cpp | Parses -fms-extensions and maps it to config.ansi_c.ms_extensions. |
| src/ansi-c/c_typecheck_type.cpp | Implements conditional acceptance of tagged anonymous struct/union members. |
| regression/goto-gcc/ms_extensions_anonymous_member/test.desc | Adds a regression test driver for the new behavior. |
| regression/goto-gcc/ms_extensions_anonymous_member/main.c | Adds a regression C file asserting layout and injected member access. |
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Comment on lines
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| // -fms-extensions enables MS/GCC extensions such as anonymous members | ||
| // that are a *tagged* struct/union type (used throughout the Linux | ||
| // kernel, e.g. struct __filename_head embedded in struct filename). | ||
| // Note: goto_cc_cmdlinet stores long options without the leading '-', | ||
| // so we query "fms-extensions" (cf. the "fshort-double" check below). | ||
| if(cmdline.isset("fms-extensions")) | ||
| config.ansi_c.ms_extensions = true; |
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| _Static_assert(sizeof(struct head) == 24, "head size"); | ||
| _Static_assert(sizeof(struct filename) == 192, "filename size"); | ||
| _Static_assert( | ||
| __builtin_offsetof(struct filename, iname) == 24, | ||
| "iname offset"); |
| }; | ||
| long tail; | ||
| }; | ||
| _Static_assert(sizeof(struct u_outer) == 16, "union-anon size"); |
Comment on lines
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| if(cmdline.isset("fms-extensions")) | ||
| config.ansi_c.ms_extensions = true; |
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## develop #9050 +/- ##
========================================
Coverage 80.68% 80.68%
========================================
Files 1714 1714
Lines 189501 189509 +8
Branches 73 73
========================================
+ Hits 152902 152912 +10
+ Misses 36599 36597 -2 ☔ View full report in Codecov by Harness. 🚀 New features to boost your workflow:
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Author
|
We'll have to consider whether to take this one or #9022. |
A *tagged* struct/union used as an unnamed (anonymous) member -- e.g. the Linux kernel's `struct __filename_head;` embedded in struct filename -- was silently dropped by the C front-end in GCC/Clang mode: it contributed no bytes (so sizeof was wrong, breaking the kernel's static_assert(sizeof(struct filename) % 64 == 0)) and its members were not injected (inaccessible by name). This is the second blocker (after __builtin_has_attribute) to compiling real kernel objects with goto-cc. GCC/Clang accept this only with -fms-extensions (without it they ignore the member, matching the existing regression/ansi-c/anonymous_member* tests). So: - config: add ansi_c.ms_extensions (default false); - gcc_mode: set it when -fms-extensions is passed (queried as "fms-extensions" -- goto_cc_cmdlinet stores long options without the leading '-'); - c_typecheck_type: in GCC/Clang mode accept an *untagged* struct/union anonymous member always (C11) and a *tagged* one only under -fms-extensions; otherwise (and for non-aggregate unnamed members) keep the previous "ignore" behaviour. With the flag the member now contributes its size and its fields are injected (reachable by name). Regression test goto-gcc/ms_extensions_anonymous_member checks exact layout via _Static_assert and member access; the existing anonymous_member1/3 (no flag, sizeof==0) confirm the default behaviour is preserved. Co-authored-by: Kiro <kiro-agent@users.noreply.github.com>
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A tagged struct/union used as an unnamed (anonymous) member -- e.g. the Linux kernel's
struct __filename_head;embedded in struct filename -- was silently dropped by the C front-end in GCC/Clang mode: it contributed no bytes (so sizeof was wrong, breaking the kernel's static_assert(sizeof(struct filename) % 64 == 0)) and its members were not injected (inaccessible by name). This is the second blocker (after __builtin_has_attribute) to compiling real kernel objects with goto-cc.GCC/Clang accept this only with -fms-extensions (without it they ignore the member, matching the existing regression/ansi-c/anonymous_member* tests). So:
With the flag the member now contributes its size and its fields are injected (reachable by name). Regression test
goto-gcc/ms_extensions_anonymous_member checks exact layout via _Static_assert and member access; the existing anonymous_member1/3 (no flag, sizeof==0) confirm the default behaviour is preserved.