[clang-cl] Map /Ot to -O3 instead of -O2

[zmodem]

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

[llvmbot]

@llvm/pr-subscribers-clang-driver

@llvm/pr-subscribers-clang

Author: Hans (zmodem)

Changes

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

---

Full diff: https://github.com/llvm/llvm-project/pull/95406.diff

5 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+4)
• (modified) clang/docs/UsersManual.rst (+2-2)
• (modified) clang/include/clang/Driver/Options.td (+2-2)
• (modified) clang/lib/Driver/ToolChains/MSVC.cpp (+2-2)
• (modified) clang/test/Driver/cl-options.c (+5-5)

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
 

[zmodem]

@AaronBallman as discussed on https://discourse.llvm.org/t/clang-cl-optimization-option/79554

Also @nico who IIRC looked at these flags in recent times.

[nico]

Can you add something like "use /O2 /clang:-O2 to restore the previous behavior" to the commit message, on case someone prefers that?

Otherwise LG.

[nico]

Oh, you have that in the release notes already, even better.

[efriedma-quic]

If we're trying to match MSVC, the amount of inlining MSVC does at /O2 is probably closer to what clang does at -O2 than -O3. Which is why it was mapped that way in 015ce0f, I think. clang's -O3 is really aggressive (which tends to look good in benchmarks, but less so in real software).

I'm a bit worried we're going to get a wave of complaints about codesize in reaction to this.

[efriedma-quic]

For reference, current MSVC has a flag /Ob3 to request more aggressive inlining.