[clang] [clang-cl] Map /Ot to -O3 instead of -O2 (PR #95406)

[via cfe-commits]

https://github.com/zmodem created https://github.com/llvm/llvm-project/pull/95406

/Ot (which is also implied by /O2) is supposed to optimize for maximum speed, so -O3 seems like a better match.

>From 90e7c202bd55fce943b77fd926f5fc4c5835dc3f Mon Sep 17 00:00:00 2001
From: Hans Wennborg <hans at chromium.org>
Date: Thu, 13 Jun 2024 15:28:55 +0200
Subject: [PATCH] [clang-cl] Map /Ot to -O3 instead of -O2

/Ot (which is also implied by /O2) is supposed to optimize for maximum
speed, so -O3 seems like a better match.
---
 clang/docs/ReleaseNotes.rst           |  4 ++++
 clang/docs/UsersManual.rst            |  4 ++--
 clang/include/clang/Driver/Options.td |  4 ++--
 clang/lib/Driver/ToolChains/MSVC.cpp  |  4 ++--
 clang/test/Driver/cl-options.c        | 10 +++++-----
 5 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 8c2f737836a9d..68355dbb5861b 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -921,6 +921,10 @@ Android Support
 Windows Support
 ^^^^^^^^^^^^^^^
 
+- The clang-cl ``/Ot`` compiler option ("optimize for speed", also implied by
+  ``/O2``) now maps to clang's ``-O3`` optimizataztion level instead of ``-O2``.
+  Users who prefer the old behavior can use ``clang-cl /Ot /clang:-O2 ...``.
+
 - Clang-cl now supports function targets with intrinsic headers. This allows
   for runtime feature detection of intrinsics. Previously under clang-cl
   ``immintrin.h`` and similar intrinsic headers would only include the intrinsics
diff --git a/clang/docs/UsersManual.rst b/clang/docs/UsersManual.rst
index f954857b0235a..ee30e4eff9ea0 100644
--- a/clang/docs/UsersManual.rst
+++ b/clang/docs/UsersManual.rst
@@ -4636,8 +4636,8 @@ Execute ``clang-cl /?`` to see a list of supported options:
       /Og                     No effect
       /Oi-                    Disable use of builtin functions
       /Oi                     Enable use of builtin functions
-      /Os                     Optimize for size
-      /Ot                     Optimize for speed
+      /Os                     Optimize for size (like clang -Os)
+      /Ot                     Optimize for speed (like clang -O3)
       /Ox                     Deprecated (same as /Og /Oi /Ot /Oy /Ob2); use /O2 instead
       /Oy-                    Disable frame pointer omission (x86 only, default)
       /Oy                     Enable frame pointer omission (x86 only)
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index 880221176027e..1620b12d55ed9 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -8279,9 +8279,9 @@ def : CLFlag<"Oi">, Alias<_SLASH_O>, AliasArgs<["i"]>,
 def : CLFlag<"Oi-">, Alias<_SLASH_O>, AliasArgs<["i-"]>,
   HelpText<"Disable use of builtin functions">;
 def : CLFlag<"Os">, Alias<_SLASH_O>, AliasArgs<["s"]>,
-  HelpText<"Optimize for size">;
+  HelpText<"Optimize for size (like clang -Os)">;
 def : CLFlag<"Ot">, Alias<_SLASH_O>, AliasArgs<["t"]>,
-  HelpText<"Optimize for speed">;
+  HelpText<"Optimize for speed (like clang -O3)">;
 def : CLFlag<"Ox">, Alias<_SLASH_O>, AliasArgs<["x"]>,
   HelpText<"Deprecated (like /Og /Oi /Ot /Oy /Ob2); use /O2">;
 def : CLFlag<"Oy">, Alias<_SLASH_O>, AliasArgs<["y"]>,
diff --git a/clang/lib/Driver/ToolChains/MSVC.cpp b/clang/lib/Driver/ToolChains/MSVC.cpp
index b7021d4b996dd..d03687208c5c6 100644
--- a/clang/lib/Driver/ToolChains/MSVC.cpp
+++ b/clang/lib/Driver/ToolChains/MSVC.cpp
@@ -861,7 +861,7 @@ static void TranslateOptArg(Arg *A, llvm::opt::DerivedArgList &DAL,
           DAL.AddJoinedArg(A, Opts.getOption(options::OPT_O), "s");
         } else if (OptChar == '2' || OptChar == 'x') {
           DAL.AddFlagArg(A, Opts.getOption(options::OPT_fbuiltin));
-          DAL.AddJoinedArg(A, Opts.getOption(options::OPT_O), "2");
+          DAL.AddJoinedArg(A, Opts.getOption(options::OPT_O), "3");
         }
         if (SupportsForcingFramePointer &&
             !DAL.hasArgNoClaim(options::OPT_fno_omit_frame_pointer))
@@ -901,7 +901,7 @@ static void TranslateOptArg(Arg *A, llvm::opt::DerivedArgList &DAL,
       DAL.AddJoinedArg(A, Opts.getOption(options::OPT_O), "s");
       break;
     case 't':
-      DAL.AddJoinedArg(A, Opts.getOption(options::OPT_O), "2");
+      DAL.AddJoinedArg(A, Opts.getOption(options::OPT_O), "3");
       break;
     case 'y': {
       bool OmitFramePointer = true;
diff --git a/clang/test/Driver/cl-options.c b/clang/test/Driver/cl-options.c
index 2c17459dde656..e77ec364170d1 100644
--- a/clang/test/Driver/cl-options.c
+++ b/clang/test/Driver/cl-options.c
@@ -189,12 +189,12 @@
 // RUN: %clang_cl /Ot --target=i686-pc-windows-msvc -### -- %s 2>&1 | FileCheck -check-prefix=Ot %s
 // RUN: %clang_cl /Ot --target=x86_64-pc-windows-msvc -### -- %s 2>&1 | FileCheck -check-prefix=Ot %s
 // Ot: -mframe-pointer=none
-// Ot: -O2
+// Ot: -O3
 
 // RUN: %clang_cl /Ox --target=i686-pc-windows-msvc -### -- %s 2>&1 | FileCheck -check-prefix=Ox %s
 // RUN: %clang_cl /Ox --target=x86_64-pc-windows-msvc -### -- %s 2>&1 | FileCheck -check-prefix=Ox %s
 // Ox: -mframe-pointer=none
-// Ox: -O2
+// Ox: -O3
 
 // RUN: %clang_cl --target=i686-pc-win32 /O2sy- -### -- %s 2>&1 | FileCheck -check-prefix=PR24003 %s
 // PR24003: -mframe-pointer=all
@@ -202,14 +202,14 @@
 
 // RUN: %clang_cl --target=i686-pc-win32 -Werror -Wno-msvc-not-found /Oy- /O2 -### -- %s 2>&1 | FileCheck -check-prefix=Oy_2 %s
 // Oy_2: -mframe-pointer=all
-// Oy_2: -O2
+// Oy_2: -O3
 
 // RUN: %clang_cl --target=aarch64-pc-windows-msvc -Werror -Wno-msvc-not-found /Oy- /O2 -### -- %s 2>&1 | FileCheck -check-prefix=Oy_aarch64 %s
 // Oy_aarch64: -mframe-pointer=non-leaf
-// Oy_aarch64: -O2
+// Oy_aarch64: -O3
 
 // RUN: %clang_cl --target=i686-pc-win32 -Werror -Wno-msvc-not-found /O2 /O2 -### -- %s 2>&1 | FileCheck -check-prefix=O2O2 %s
-// O2O2: "-O2"
+// O2O2: "-O3"
 
 // RUN: %clang_cl /Zs -Werror /Oy -- %s 2>&1