[llvm] [DebugInfo] Truncate implicit value constants to source type width (PR #206671)

[via llvm-commits]

https://github.com/firmiana402 created https://github.com/llvm/llvm-project/pull/206671

This is a follow-up to #204353.

@mikaelholmen and @bevin-hansson reported that the previous change could assert downstream when emitting `DW_OP_implicit_value` for a source integer type wider than the target generic DWARF stack type, if the debug-value carrier integer contains bits outside the declared source type width.

The fix is to construct the source-width `APInt` with explicit truncation enabled before emitting the implicit value bytes. This preserves the intended wrap/truncate behavior and avoids asserting on otherwise recoverable debug-value input.

A regression test is added for an `unsigned _BitInt(48)` debug value on i386, covering both an out-of-range positive carrier value and an all-ones negative carrier value.

>From 78addeb0930fc29a28d2063b8e54b4809f0bea90 Mon Sep 17 00:00:00 2001
From: firmiana402 <firmiana402 at gmail.com>
Date: Tue, 30 Jun 2026 15:38:24 +0800
Subject: [PATCH] [DebugInfo] Truncate implicit value constants to source type
 width

---
 llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp    |  3 +-
 .../X86/implicit-value-truncated-integer.ll   | 38 +++++++++++++++++++
 2 files changed, 40 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/DebugInfo/X86/implicit-value-truncated-integer.ll

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index 64b6b27346575..314b417507af6 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -3278,7 +3278,8 @@ void DwarfDebug::emitDebugLocValue(const AsmPrinter &AP, const DIBasicType *BT,
         if (TypeBitSize > GenericBitSize && IsByteSized && IsOutOfRange) {
           DwarfExpr.addImplicitValue(
               APInt(static_cast<unsigned>(TypeBitSize),
-                    static_cast<uint64_t>(Entry.getInt()), IsSigned),
+                    static_cast<uint64_t>(Entry.getInt()), IsSigned,
+                    /*implicitTrunc=*/true),
               AP);
           return true;
         }
diff --git a/llvm/test/DebugInfo/X86/implicit-value-truncated-integer.ll b/llvm/test/DebugInfo/X86/implicit-value-truncated-integer.ll
new file mode 100644
index 0000000000000..3f6d238a4dffc
--- /dev/null
+++ b/llvm/test/DebugInfo/X86/implicit-value-truncated-integer.ll
@@ -0,0 +1,38 @@
+; RUN: llc -mtriple=i386-unknown-linux-gnu -filetype=obj %s -o - | llvm-dwarfdump -v -debug-info - | FileCheck %s
+
+; A constant debug value may use a carrier integer whose bits do not fit in the
+; declared source type. When emitting source-sized DW_OP_implicit_value bytes,
+; truncate the carrier value to the source type instead of asserting.
+
+; CHECK: DW_TAG_variable
+; CHECK-NEXT: DW_AT_location
+; CHECK: DW_OP_implicit_value 0x6 0x01 0x00 0x00 0x00 0x00 0x00
+; CHECK: DW_OP_implicit_value 0x6 0xff 0xff 0xff 0xff 0xff 0xff
+; CHECK: DW_AT_name {{.*}}"x"
+
+target triple = "i386-unknown-linux-gnu"
+
+define void @test(ptr %p) !dbg !4 {
+entry:
+  #dbg_value(i64 281474976710657, !6, !DIExpression(), !8)
+  store volatile i32 0, ptr %p, !dbg !8
+  #dbg_value(i64 -1, !6, !DIExpression(), !9)
+  store volatile i32 1, ptr %p, !dbg !9
+  ret void, !dbg !10
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !11}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C11, file: !1, isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+!1 = !DIFile(filename: "t.c", directory: "/tmp")
+!2 = !{}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = distinct !DISubprogram(name: "test", scope: !1, file: !1, type: !5, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0)
+!5 = !DISubroutineType(types: !2)
+!6 = !DILocalVariable(name: "x", scope: !4, file: !1, type: !7)
+!7 = !DIBasicType(name: "unsigned _BitInt(48)", size: 48, encoding: DW_ATE_unsigned)
+!8 = !DILocation(line: 1, scope: !4)
+!9 = !DILocation(line: 2, scope: !4)
+!10 = !DILocation(line: 3, scope: !4)
+!11 = !{i32 7, !"Dwarf Version", i32 4}