https://github.com/huihzhang created
https://github.com/llvm/llvm-project/pull/68460
Add an user note to pass -fstrict-aliasing explicitly for clang-cl at
optimization levels O3 and below to enable TBAA.
Also add help text for -f[no-]strict-aliasing.
>From 866ebc3da4208f2be206cc9839fc06d689d9bf34 Mon Sep 17 00:00:00 2001
From: Huihui Zhang <huih...@quicinc.com>
Date: Fri, 6 Oct 2023 17:37:03 -0700
Subject: [PATCH] [clang-cl] Document behavior difference of strict aliasing in
clang-cl vs clang.
Add an user note to pass -fstrict-aliasing explicitly for clang-cl at
optimization levels O3 and below to enable TBAA.
Also add help text for -f[no-]strict-aliasing.
---
clang/docs/UsersManual.rst | 16 ++++++++++++++++
clang/include/clang/Driver/Options.td | 6 ++++--
2 files changed, 20 insertions(+), 2 deletions(-)
diff --git a/clang/docs/UsersManual.rst b/clang/docs/UsersManual.rst
index 25bbd72c81f7a3b..704a634ffd8c8c4 100644
--- a/clang/docs/UsersManual.rst
+++ b/clang/docs/UsersManual.rst
@@ -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
@@ -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
@@ -4722,3 +4724,17 @@ 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 turned off by default in clang-cl at optimization
+levels O3 and below, and turned on at Ofast. 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 when compiling at O3 and below.
diff --git a/clang/include/clang/Driver/Options.td
b/clang/include/clang/Driver/Options.td
index 5415b18d3f406df..2109513364dba14 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -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 (default)">;
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>;
@@ -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 "
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