[clang][bytecode] Implement the constexpr built-in abs function.

[c8ef]

The current built-in abs function cannot be used in a constexpr environment. This patch fixes the issue.
See also: https://godbolt.org/z/s8x3Yd8ne.

[llvmbot]

@llvm/pr-subscribers-clang

Author: None (c8ef)

Changes

The current built-in abs function cannot be used in a constexpr environment. This patch fixes the issue.
See also: https://godbolt.org/z/s8x3Yd8ne.

---

Full diff: https://github.com/llvm/llvm-project/pull/112459.diff

3 Files Affected:

• (modified) clang/include/clang/Basic/Builtins.td (+1)
• (modified) clang/lib/AST/ExprConstant.cpp (+11)
• (modified) clang/test/AST/ByteCode/builtin-functions.cpp (+9)

diff --git a/clang/include/clang/Basic/Builtins.td b/clang/include/clang/Basic/Builtins.td
index bda8a48be92bda..e1b4d5b1fdc0a5 100644
--- a/clang/include/clang/Basic/Builtins.td
+++ b/clang/include/clang/Basic/Builtins.td
@@ -2714,6 +2714,7 @@ def Abs : IntMathTemplate, LibBuiltin<"stdlib.h"> {
   let Attributes = [NoThrow, Const];
   let Prototype = "T(T)";
   let AddBuiltinPrefixedAlias = 1;
+  let OnlyBuiltinPrefixedAliasIsConstexpr = 1;
 }
 
 def Calloc : LibBuiltin<"stdlib.h"> {
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 52a7f5778ce6d2..69539a7f03feba 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -13055,6 +13055,17 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
     return Success(Val.popcount() % 2, E);
   }
 
+  case Builtin::BI__builtin_abs:
+  case Builtin::BI__builtin_labs:
+  case Builtin::BI__builtin_llabs: {
+    APSInt Val;
+    if (!EvaluateInteger(E->getArg(0), Val, Info))
+      return false;
+    if (Val.isNegative())
+      Val.negate();
+    return Success(Val, E);
+  }
+
   case Builtin::BI__builtin_popcount:
   case Builtin::BI__builtin_popcountl:
   case Builtin::BI__builtin_popcountll:
diff --git a/clang/test/AST/ByteCode/builtin-functions.cpp b/clang/test/AST/ByteCode/builtin-functions.cpp
index 450ff5671314db..46e5b0579bd575 100644
--- a/clang/test/AST/ByteCode/builtin-functions.cpp
+++ b/clang/test/AST/ByteCode/builtin-functions.cpp
@@ -265,6 +265,15 @@ namespace fpclassify {
   char classify_subnorm [__builtin_fpclassify(-1, -1, -1, +1, -1, 1.0e-38f)];
 }
 
+namespace abs {
+static_assert(__builtin_abs(14) == 14, "");
+static_assert(__builtin_labs(14L) == 14L, "");
+static_assert(__builtin_llabs(14LL) == 14LL, "");
+static_assert(__builtin_abs(-14) == 14, "");
+static_assert(__builtin_labs(-14L) == 14L, "");
+static_assert(__builtin_llabs(-14LL) == 14LL, "");
+} // namespace abs
+
 namespace fabs {
   static_assert(__builtin_fabs(-14.0) == 14.0, "");
 }

[tbaederr]

This patch fixes the issue.

What issue? Is there a bug open about this?

You didn't implement it for the bytecode interpreter at all, only for the current interpreter.

[c8ef]

This patch fixes the issue.

What issue? Is there a bug open about this?

You didn't implement it for the bytecode interpreter at all, only for the current interpreter.

Yes, I realized that. Apologies for the oversight. I will add the implementation to InterpBuiltin.cpp.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[c8ef]

This issue arose when I was trying to address #51787, which concerns vector operations built-ins. I wanted to see how the single versions were implemented, only to discover that __builtin_abs cannot be evaluated in a constexpr context. So, I tried to fix it.