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[clang-cl] Document behavior difference of strict aliasing in clang-cl vs clang. #68460

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Oct 9, 2023
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[clang-cl] Document behavior difference of strict aliasing in clang-cl vs clang. #68460

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[clang-cl] Document behavior difference of strict aliasing in clang-c…
huihzhang Oct 7, 2023
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[clang-cl] Document behavior difference of strict aliasing in clang-c…
huihzhang Oct 9, 2023
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15 changes: 15 additions & 0 deletions clang/docs/UsersManual.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -4383,6 +4383,7 @@ Execute ``clang-cl /?`` to see a list of supported options:
-fno-sanitize-trap=<value>
Disable trapping for specified sanitizers
-fno-standalone-debug Limit debug information produced to reduce size of debug binary
-fno-strict-aliasing Disable optimizations based on strict aliasing rules (default)
-fobjc-runtime=<value> Specify the target Objective-C runtime kind and version
-fprofile-exclude-files=<value>
Instrument only functions from files where names don't match all the regexes separated by a semi-colon
Expand Down Expand Up @@ -4444,6 +4445,7 @@ Execute ``clang-cl /?`` to see a list of supported options:
behavior. See user manual for available checks
-fsplit-lto-unit Enables splitting of the LTO unit.
-fstandalone-debug Emit full debug info for all types used by the program
-fstrict-aliasing Enable optimizations based on strict aliasing rules
-fsyntax-only Run the preprocessor, parser and semantic analysis stages
-fwhole-program-vtables Enables whole-program vtable optimization. Requires -flto
-gcodeview-ghash Emit type record hashes in a .debug$H section
Expand Down Expand Up @@ -4722,3 +4724,16 @@ The Visual C++ Toolset has a slightly more elaborate mechanism for detection.
The registry information is used to help locate the installation as a final
fallback. This is only possible for pre-VS2017 installations and is
considered deprecated.

Restrictions and Limitations compared to Clang
----------------------------------------------

Strict Aliasing
^^^^^^^^^^^^^^^

Strict aliasing (TBAA) is always off by default in clang-cl. Whereas in clang,
strict aliasing is turned on by default for all optimization levels.

To enable LLVM optimizations based on strict aliasing rules (e.g., optimizations
based on type of expressions in C/C++), user will need to explicitly pass
`-fstrict-aliasing` to clang-cl.
6 changes: 4 additions & 2 deletions clang/include/clang/Driver/Options.td
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Expand Up @@ -3152,7 +3152,8 @@ defm diagnostics_show_line_numbers : BoolFOption<"diagnostics-show-line-numbers"
def fno_stack_protector : Flag<["-"], "fno-stack-protector">, Group<f_Group>,
HelpText<"Disable the use of stack protectors">;
def fno_strict_aliasing : Flag<["-"], "fno-strict-aliasing">, Group<f_Group>,
Flags<[NoXarchOption]>, Visibility<[ClangOption, CLOption, DXCOption]>;
Flags<[NoXarchOption]>, Visibility<[ClangOption, CLOption, DXCOption]>,
HelpText<"Disable optimizations based on strict aliasing rules">;
def fstruct_path_tbaa : Flag<["-"], "fstruct-path-tbaa">, Group<f_Group>;
def fno_struct_path_tbaa : Flag<["-"], "fno-struct-path-tbaa">, Group<f_Group>;
def fno_strict_enums : Flag<["-"], "fno-strict-enums">, Group<f_Group>;
Expand Down Expand Up @@ -3595,7 +3596,8 @@ def fno_debug_macro : Flag<["-"], "fno-debug-macro">, Group<f_Group>,
Visibility<[ClangOption, CLOption, DXCOption]>,
HelpText<"Do not emit macro debug information">;
def fstrict_aliasing : Flag<["-"], "fstrict-aliasing">, Group<f_Group>,
Flags<[NoXarchOption]>, Visibility<[ClangOption, CLOption, DXCOption]>;
Flags<[NoXarchOption]>, Visibility<[ClangOption, CLOption, DXCOption]>,
HelpText<"Enable optimizations based on strict aliasing rules">;
def fstrict_enums : Flag<["-"], "fstrict-enums">, Group<f_Group>,
Visibility<[ClangOption, CC1Option]>,
HelpText<"Enable optimizations based on the strict definition of an enum's "
Expand Down