[Clang] Add __builtin_bswapg #162433

clingfei · 2025-10-08T07:14:30Z

Add a new builtin function __builtin_bswapg. It works on any integral types that has a multiple of 16 bits as well as a single byte.

Closes #160266

github-actions · 2025-10-08T07:14:46Z

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llvmbot · 2025-10-08T07:15:21Z

@llvm/pr-subscribers-clang-codegen

@llvm/pr-subscribers-clang

Author: clf (clingfei)

Changes

Add a new builtin function __builtin_bswapg. It works on any integral types that has a multiple of 16 bits as well as a single byte, as described in #160266.

Full diff: https://github.com/llvm/llvm-project/pull/162433.diff

3 Files Affected:

(modified) clang/include/clang/Basic/Builtins.td (+6)
(modified) clang/lib/AST/ExprConstant.cpp (+11)
(modified) clang/lib/Sema/SemaChecking.cpp (+28)

diff --git a/clang/include/clang/Basic/Builtins.td b/clang/include/clang/Basic/Builtins.td
index 468121f7d20ab..e65ed2f20be97 100644
--- a/clang/include/clang/Basic/Builtins.td
+++ b/clang/include/clang/Basic/Builtins.td
@@ -755,6 +755,12 @@ def BSwap : Builtin, Template<["unsigned short", "uint32_t", "uint64_t"],
   let Prototype = "T(T)";
 }
 
+def BSwapg : Builtin {
+  let Spellings = ["__builtin_bswapg"];
+  let Attributes = [NoThrow, Const, Constexpr, CustomTypeChecking];
+  let Prototype = "int(...)";
+}
+
 def Bitreverse : BitInt8_16_32_64BuiltinsTemplate, Builtin {
   let Spellings = ["__builtin_bitreverse"];
   let Attributes = [NoThrow, Const, Constexpr];
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 618e1636e9e53..058905e7fd3c0 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -13982,6 +13982,17 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
 
     return Success(Val.reverseBits(), E);
   }
+  case Builtin::BI__builtin_bswapg: {
+    APSInt Val;
+    if (!EvaluateInteger(E->getArg(0), Val, Info))
+      return false;
+    if (Val.getBitWidth() == 8) {
+        bool ret =  Success(Val, E);
+        return ret;
+    }
+        
+    return Success(Val.byteSwap(), E);
+  }
 
   case Builtin::BI__builtin_bswap16:
   case Builtin::BI__builtin_bswap32:
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 063db05665af1..362b53676feaa 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -2200,6 +2200,30 @@ static bool BuiltinCpu(Sema &S, const TargetInfo &TI, CallExpr *TheCall,
   return false;
 }
 
+/// Checks that __builtin_bswapg was called with a single argument, which is an
+/// unsigned integer, and overrides the return value type to the integer type.
+static bool BuiltinBswapg(Sema &S, CallExpr *TheCall) {
+  if (S.checkArgCount(TheCall, 1))
+    return true;
+  ExprResult ArgRes = S.DefaultLvalueConversion(TheCall->getArg(0));
+  if (ArgRes.isInvalid())
+    return true;
+
+  Expr *Arg = ArgRes.get();
+  TheCall->setArg(0, Arg);
+
+  QualType ArgTy = Arg->getType();
+
+  if (!ArgTy->isIntegerType()) {
+    S.Diag(Arg->getBeginLoc(), diag::err_builtin_invalid_arg_type)
+        << 1 << /* scalar */ 1 << /* unsigned integer ty */ 1 << /* no fp */ 0
+        << ArgTy;
+    return true;
+  }
+  TheCall->setType(ArgTy);
+  return false;
+}
+
 /// Checks that __builtin_popcountg was called with a single argument, which is
 /// an unsigned integer.
 static bool BuiltinPopcountg(Sema &S, CallExpr *TheCall) {
@@ -3448,6 +3472,10 @@ Sema::CheckBuiltinFunctionCall(FunctionDecl *FDecl, unsigned BuiltinID,
     }
     break;
   }
+  case Builtin::BI__builtin_bswapg:
+    if (BuiltinBswapg(*this, TheCall))
+        return ExprError();
+    break;
   case Builtin::BI__builtin_popcountg:
     if (BuiltinPopcountg(*this, TheCall))
       return ExprError();

clang/lib/AST/ByteCode/InterpBuiltin.cpp

ojhunt

I'd think this is something that @AaronBallman or similar should approve (spelling at least).

There should be actual documentation about this in LanguageExtensions.rst (it seems like like the existing ones don't have documentation, but maybe they should be placed into a group just documenting the family of builtins), and a note in ReleaseNotes.rst

clang/lib/AST/ExprConstant.cpp

clang/lib/Sema/SemaChecking.cpp

… VisitBuiltinCallExpr

efriedma-quic

Do we want to allow bswap of _BitInt values?

github-actions · 2025-10-09T17:20:26Z

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff origin/main HEAD --extensions cpp,c -- clang/test/SemaCXX/builtin-bswapg.cpp clang/lib/AST/ByteCode/InterpBuiltin.cpp clang/lib/AST/ExprConstant.cpp clang/lib/CodeGen/CGBuiltin.cpp clang/lib/Sema/SemaChecking.cpp clang/test/AST/ByteCode/builtin-functions.cpp clang/test/CodeGen/builtins.c clang/test/CodeGenCXX/builtins.cpp clang/test/Sema/constant-builtins-2.c clang/test/Sema/constant-builtins.c --diff_from_common_commit

⚠️
The reproduction instructions above might return results for more than one PR
in a stack if you are using a stacked PR workflow. You can limit the results by
changing origin/main to the base branch/commit you want to compare against.
⚠️

View the diff from clang-format here.

diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 38b9e0fed..428cf1be0 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -2218,7 +2218,7 @@ static bool BuiltinBswapg(Sema &S, CallExpr *TheCall) {
 
   if (!ArgTy->isIntegerType()) {
     S.Diag(Arg->getBeginLoc(), diag::err_builtin_invalid_arg_type)
-        << 1 << /*scalar=*/ 1 << /*unsigned integer=*/ 1 << /*floating point=*/ 0
+        << 1 << /*scalar=*/1 << /*unsigned integer=*/1 << /*floating point=*/0
         << ArgTy;
     return true;
   }

clang/test/Sema/constant-builtins-2.c

clang/test/CodeGen/builtins.c

ojhunt · 2025-10-10T07:11:02Z

I accidentally added a large number of unintended reviewers. Could you help me remove them?

I believe I've got it back to the right set :D

How did you update your branch? (My suspicion is that something about that caused GitHub to suggest every person involved in all the merged PRs)

clingfei · 2025-10-10T07:14:56Z

I accidentally added a large number of unintended reviewers. Could you help me remove them?

I believe I've got it back to the right set :D

Thanks!

How did you update your branch? (My suspicion is that something about that caused GitHub to suggest every person involved in all the merged PRs)

I performed a local rebase onto the main branch and pushed to my own branch.

philnik777

This seems to be missing testing for _BitInt.

clang/test/Sema/builtin-bswapg.cpp

clang/test/CodeGen/builtins.c

clang/lib/Sema/SemaChecking.cpp

clang/lib/AST/ExprConstant.cpp

ojhunt · 2025-10-10T07:35:35Z

clang/lib/CodeGen/CGBuiltin.cpp

+  case Builtin::BI__builtin_bswapg: {
+    Value *ArgValue = EmitScalarExpr(E->getArg(0));
+    llvm::IntegerType *IntTy = cast<llvm::IntegerType>(ArgValue->getType());
+    assert(IntTy && "LLVM's __builtin_bswapg only supports integer variants");


This should probably have a getBitWidth() % 8 == 0 and getBitWidth() > 0, as a defense for future support of _BitInt if we don't simply include that now (which I think someone has already asked about?)

As @philnik777 described in #160266, bswapg should work on any integral types that have a multiple of 16 bits as well as a single byte. Thus, the condition should be (getBitWidth() % 16 == 0 and getBitWidth() > 0) or getBitWidth() == 8?

I don't think mod 16 is right either, swapping _BitInt(24) (etc) is in principle just as reasonable a swap, though it would be a novel choice. And at the same time as disallowing _BitInt(24), the mod 16 rule allows the equivalently odd _BitInt(48) to be swapped.

Would it be more reasonable to simply say it must be a power of 2 number of bytes (N=8*2^^N for some positive N)? @philnik777 does that sound reasonable?

It sounds reasonable, but the llvm intrinsic has the constraint on 16 bits. I don't think we should block introducing the builtin on extending the llvm intrinsic.

ojhunt · 2025-10-10T07:43:14Z

I accidentally added a large number of unintended reviewers. Could you help me remove them?

I believe I've got it back to the right set :D

Thanks!

How did you update your branch? (My suspicion is that something about that caused GitHub to suggest every person involved in all the merged PRs)

I performed a local rebase onto the main branch and pushed to my own branch.

It's probably just GH being weird then - it doesn't do well with rebasing. I haven't seen this particular result before, but that doesn't mean it won't. I believe GH prefers merging main into your branch (though I agree that rebasing is much more sensible).

… conversation

shafik

Can we get a release note.

shafik · 2025-10-13T21:07:30Z

clang/test/Sema/constant-builtins-2.c

+int h7 = __builtin_bswapg((short)(0x1234)) == (short)(0x3412) ? 1 : f();
+int h8 = __builtin_bswapg(0x00001234) == 0x34120000 ? 1 : f();
+int h9 = __builtin_bswapg(0x0000000000001234ULL) == 0x3412000000000000 ? 1 : f();
+float h10 = __builtin_bswapg(1.0f); // expected-error {{1st argument must be a scalar integer type (was 'float')}}


How about also testing: arrays, pointers, enums, class, nullptr_t.

What do we expect w/ charN_t, wchar_t, bool and int128_t types?

Testing for arrays, pointers, enums, class, nullptr_t has been added.

What do we expect w/ charN_t, wchar_t, bool and int128_t types?

Currently __builtin_bswapg treats these types the same way as the corresponding byte integer types.

Thank you for the additional test, should we add a _BitInt test @AaronBallman

clingfei · 2025-10-18T08:40:36Z

The test for _BitInt has been added.

ojhunt · 2025-10-19T04:46:41Z

clang/lib/CodeGen/CGBuiltin.cpp

+  case Builtin::BI__builtin_bswapg: {
+    Value *ArgValue = EmitScalarExpr(E->getArg(0));
+    llvm::IntegerType *IntTy = cast<llvm::IntegerType>(ArgValue->getType());
+    assert(IntTy && "LLVM's __builtin_bswapg only supports integer variants");


I don't think mod 16 is right either, swapping _BitInt(24) (etc) is in principle just as reasonable a swap, though it would be a novel choice. And at the same time as disallowing _BitInt(24), the mod 16 rule allows the equivalently odd _BitInt(48) to be swapped.

Would it be more reasonable to simply say it must be a power of 2 number of bytes (N=8*2^^N for some positive N)? @philnik777 does that sound reasonable?

clang/test/SemaCXX/builtin-bswapg.cpp

ojhunt

This looks good to me sans the mod 16 bit rule vs 8*2^^N question

clang/test/CodeGen/builtins.c

…wap to support bswap for values with more bits

llvmbot added clang Clang issues not falling into any other category clang:frontend Language frontend issues, e.g. anything involving "Sema" labels Oct 8, 2025

clingfei force-pushed the builtin branch from 3b773c9 to f74dc63 Compare October 8, 2025 07:31

[Clang] Add __builtin_bswapg

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clingfei force-pushed the builtin branch from f74dc63 to 92466f3 Compare October 8, 2025 09:12

llvmbot added clang:codegen IR generation bugs: mangling, exceptions, etc. clang:bytecode Issues for the clang bytecode constexpr interpreter labels Oct 8, 2025

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clingfei added 2 commits October 9, 2025 13:14

handle special case 8 in interp__builtin_bswap and fix extra space in…

d4ea868

… VisitBuiltinCallExpr

add dependent check for argument

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fix test for builtin-bswapg

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add tests for arrays, pointers, enums, class, nullptr_t

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add release notes

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add CodeGen tests for _BitInt

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remove the assumption 'Val.getActiveBits <= 64' in interp__builtin_bs…

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…wap to support bswap for values with more bits

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