[clang] Support constexpr alignment of __builtin_assume_aligned.

[tangjj11]

No description provided.

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[llvmbot]

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-codegen

Author: Tang Jiajun (tangjj11)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/127223.diff

2 Files Affected:

• (modified) clang/lib/CodeGen/CGBuiltin.cpp (+12-8)
• (added) clang/test/CodeGen/builtin-assume-aligned.cpp (+17)

diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index d57f491a20c8e..bff5451a01300 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -3829,14 +3829,18 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
       (E->getNumArgs() > 2) ? EmitScalarExpr(E->getArg(2)) : nullptr;
 
     Value *AlignmentValue = EmitScalarExpr(E->getArg(1));
-    ConstantInt *AlignmentCI = cast<ConstantInt>(AlignmentValue);
-    if (AlignmentCI->getValue().ugt(llvm::Value::MaximumAlignment))
-      AlignmentCI = ConstantInt::get(AlignmentCI->getIntegerType(),
-                                     llvm::Value::MaximumAlignment);
-
-    emitAlignmentAssumption(PtrValue, Ptr,
-                            /*The expr loc is sufficient.*/ SourceLocation(),
-                            AlignmentCI, OffsetValue);
+    if (ConstantInt *AlignmentCI = cast<ConstantInt>(AlignmentValue)) {
+      if (AlignmentCI->getValue().ugt(llvm::Value::MaximumAlignment))
+        AlignmentCI = ConstantInt::get(AlignmentCI->getIntegerType(),
+                                       llvm::Value::MaximumAlignment);
+
+      emitAlignmentAssumption(PtrValue, Ptr,
+                              /*The expr loc is sufficient.*/ SourceLocation(),
+                              AlignmentCI, OffsetValue);
+    } else
+      emitAlignmentAssumption(PtrValue, Ptr,
+                              /*The expr loc is sufficient.*/ SourceLocation(),
+                              AlignmentValue, OffsetValue);
     return RValue::get(PtrValue);
   }
   case Builtin::BI__builtin_assume_dereferenceable: {
diff --git a/clang/test/CodeGen/builtin-assume-aligned.cpp b/clang/test/CodeGen/builtin-assume-aligned.cpp
new file mode 100644
index 0000000000000..fae4221d7d7e7
--- /dev/null
+++ b/clang/test/CodeGen/builtin-assume-aligned.cpp
@@ -0,0 +1,17 @@
+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py
+// RUN: %clang_cc1 -triple x86_64-unknown-unknown -emit-llvm -o - %s | FileCheck %s
+
+constexpr int get_align() { return 1; }
+
+// CHECK-LABEL: @Ztest1P(
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    [[A_ADDR:%.*]] = alloca ptr, align 8
+// CHECK-NEXT:    store ptr [[A:%.*]], ptr [[A_ADDR]], align 8
+// CHECK-NEXT:    [[TMP0:%.*]] = load ptr, ptr [[A_ADDR]], align 8
+// CHECK-NEXT:    call void @llvm.assume(i1 true) [ "align"(ptr [[TMP0]], i64 32, i64 0) ]
+// CHECK-NEXT:    store ptr [[TMP0]], ptr [[A_ADDR]], align 8
+// CHECK-NEXT:    [[TMP3:%.*]] = load ptr, ptr [[A_ADDR]], align 8
+// CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds i32, ptr [[TMP3]], i64 0
+// CHECK-NEXT:    [[TMP4:%.*]] = load i32, ptr [[ARRAYIDX]], align 4
+// CHECK-NEXT:    ret i32 [[TMP4]]
+void *test1(void *a) { return __builtin_assume_aligned(a, 32, get_align()); }

[efriedma-quic]

This is not the right fix.

After we check the constant expression in Sema, we should create a ConstantExpr containing the the evaluated value, so we don't need to re-evaluate it later.

[tangjj11]

This is not the right fix.

After we check the constant expression in Sema, we should create a ConstantExpr containing the the evaluated value, so we don't need to re-evaluate it later.

thanks, I will close the PR.