First draft: [Clang] Add constant evaluation support for x86 psadbw/p…

[Mohxen]

This is the first draft of adding constant evaluation support for the
x86 PSADBW and PSADBW128 intrinsics in Clang’s constexpr interpreter.

The patch currently:

• Adds initial handling logic in ExprConstant.cpp
• Includes an early version of the test under
  clang/test/AST/ByteCode/x86-psadbw-psadbw128.cpp

This PR is still under development. Feedback on structure, integration
location, and testing is welcome before polishing the final version.

Partial fix: #157522

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⚠️ 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 -- clang/test/AST/ByteCode/x86-psadbw-psadbw128.cpp clang/lib/AST/ExprConstant.cpp --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/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 3a41b262d..e862ba7b7 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -1901,7 +1901,8 @@ static bool EvaluateFixedPointOrInteger(const Expr *E, APFixedPoint &Result,
 static bool EvaluateFixedPoint(const Expr *E, APFixedPoint &Result,
                                EvalInfo &Info);
 
-static bool EvaluatePSADBW128(const CallExpr *E, EvalInfo &Info, APValue &Result);
+static bool EvaluatePSADBW128(const CallExpr *E, EvalInfo &Info,
+                              APValue &Result);
 
 //===----------------------------------------------------------------------===//
 // Misc utilities
@@ -12080,11 +12081,11 @@ static bool evalPackBuiltin(const CallExpr *E, EvalInfo &Info, APValue &Result,
   return true;
 }
 
-static bool EvaluatePSADBW128(const CallExpr *E, EvalInfo &Info, APValue &Result) {
+static bool EvaluatePSADBW128(const CallExpr *E, EvalInfo &Info,
+                              APValue &Result) {
   // 1) Evaluate the arguments into APValues
   APValue A, B;
-  if (!Evaluate(A, Info, E->getArg(0)) ||
-      !Evaluate(B, Info, E->getArg(1)))
+  if (!Evaluate(A, Info, E->getArg(0)) || !Evaluate(B, Info, E->getArg(1)))
     return false;
 
   if (!A.isVector() || !B.isVector())
@@ -12127,7 +12128,6 @@ static bool EvaluatePSADBW128(const CallExpr *E, EvalInfo &Info, APValue &Result
   return true;
 }
 
-
 static bool evalShuffleGeneric(
     EvalInfo &Info, const CallExpr *Call, APValue &Out,
     llvm::function_ref<std::pair<unsigned, int>(unsigned, unsigned)>
@@ -12411,7 +12411,7 @@ bool VectorExprEvaluator::VisitCallExpr(const CallExpr *E) {
     return EvaluateBinOpExpr(llvm::APIntOps::avgCeilU);
 
   case X86::BI__builtin_ia32_psadbw128:
-  return EvaluatePSADBW128(E, Info, Result);
+    return EvaluatePSADBW128(E, Info, Result);
 
   case clang::X86::BI__builtin_ia32_pmulhrsw128:
   case clang::X86::BI__builtin_ia32_pmulhrsw256:

[RKSimon]

Use APIntOps::abdu ?

[RKSimon]

The ia32_psadbw builtins have fixed types - don't bother with so much isVector/getVectorLength checks - assert((getVectorLength() % 16) == 0) should be enough.

[RKSimon]

Convert to a format that will handle 256/512 variants now - otherwise you'll just end up rewriting all of this later on.

SmallVector<APValue, 8> Elts;
for (unsigned Lane = 0; Lane != Len; Lane += 8) {
  for (unsigend I = 0; I != 8; ++I) {
    APInt A = A.getVectorElt(Lane + I).getInt();
    APInt B = B.getVectorElt(Lane + I).getInt();
  }
etc.
  Elts.emplace_back(APValue(APSInt(APInt(64, Sum), Unsigned)));
}

[github-actions]

🐧 Linux x64 Test Results

• 84729 tests passed
• 1108 tests skipped
• 1 test failed

Failed Tests

(click on a test name to see its output)

Clang

Clang.AST/ByteCode/x86-psadbw-psadbw128.cpp

Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
/home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/clang -cc1 -internal-isystem /home/gha/actions-runner/_work/llvm-project/llvm-project/build/lib/clang/22/include -nostdsysteminc -std=c++20 -triple x86_64-unknown-linux-gnu -O0    -fexperimental-new-constant-interpreter -verify /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/AST/ByteCode/x86-psadbw-psadbw128.cpp
# executed command: /home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/clang -cc1 -internal-isystem /home/gha/actions-runner/_work/llvm-project/llvm-project/build/lib/clang/22/include -nostdsysteminc -std=c++20 -triple x86_64-unknown-linux-gnu -O0 -fexperimental-new-constant-interpreter -verify /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/AST/ByteCode/x86-psadbw-psadbw128.cpp
# .---command stderr------------
# | error: no expected directives found: consider use of 'expected-no-diagnostics'
# | error: 'expected-error' diagnostics seen but not expected: 
# |   File /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/AST/ByteCode/x86-psadbw-psadbw128.cpp Line 20: static assertion expression is not an integral constant expression
# |   File /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/AST/ByteCode/x86-psadbw-psadbw128.cpp Line 21: static assertion expression is not an integral constant expression
# | error: 'expected-note' diagnostics seen but not expected: 
# |   File /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/AST/ByteCode/x86-psadbw-psadbw128.cpp Line 17: subexpression not valid in a constant expression
# |   File /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/AST/ByteCode/x86-psadbw-psadbw128.cpp Line 20: in call to 'test_psadbw128()'
# |   File /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/AST/ByteCode/x86-psadbw-psadbw128.cpp Line 17: subexpression not valid in a constant expression
# |   File /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/AST/ByteCode/x86-psadbw-psadbw128.cpp Line 21: in call to 'test_psadbw128()'
# | 7 errors generated.
# `-----------------------------
# error: command failed with exit status: 1

--

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the infrastructure label.