diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -164,6 +164,8 @@
   New->takeName(&AI);
   New->setUsedWithInAlloca(AI.isUsedWithInAlloca());
 
+  replaceAllDbgUsesWith(AI, *New, *New, DT);
+
   // If the allocation has multiple real uses, insert a cast and change all
   // things that used it to use the new cast.  This will also hack on CI, but it
   // will die soon.
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
--- a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
@@ -165,7 +165,7 @@
 }
 
 static Instruction *simplifyAllocaArraySize(InstCombinerImpl &IC,
-                                            AllocaInst &AI) {
+                                            AllocaInst &AI, DominatorTree &DT) {
   // Check for array size of 1 (scalar allocation).
   if (!AI.isArrayAllocation()) {
     // i32 1 is the canonical array size for scalar allocations.
@@ -184,6 +184,8 @@
                                                 nullptr, AI.getName());
       New->setAlignment(AI.getAlign());
 
+      replaceAllDbgUsesWith(AI, *New, *New, DT);
+
       // Scan to the end of the allocation instructions, to skip over a block of
       // allocas if possible...also skip interleaved debug info
       //
@@ -352,7 +354,7 @@
 }
 
 Instruction *InstCombinerImpl::visitAllocaInst(AllocaInst &AI) {
-  if (auto *I = simplifyAllocaArraySize(*this, AI))
+  if (auto *I = simplifyAllocaArraySize(*this, AI, DT))
     return I;
 
   if (AI.getAllocatedType()->isSized()) {
diff --git a/llvm/test/Transforms/InstCombine/no-undef-bitcast-to-gep-ret.ll b/llvm/test/Transforms/InstCombine/no-undef-bitcast-to-gep-ret.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/no-undef-bitcast-to-gep-ret.ll
@@ -0,0 +1,44 @@
+; RUN: opt -S -instcombine %s | FileCheck %s
+
+; https://github.com/llvm/llvm-project/issues/56807
+%struct.Pixel = type { i8, i8, i8 }
+
+declare void @llvm.dbg.declare(metadata, metadata, metadata)
+
+; CHECK-LABEL: define dso_local i8* @toplevel(
+; CHECK:  entry:
+; CHECK-NEXT:    %pixels1 = alloca [1500 x i8], align 8
+; CHECK-NEXT:    call void @llvm.dbg.declare(metadata [1500 x i8]* %pixels1, metadata !7, metadata !DIExpression()), !dbg !12
+; CHECK-NEXT:    %pixels1.sub = getelementptr inbounds [1500 x i8], [1500 x i8]* %pixels1, i64 0, i64 0
+; CHECK-NEXT:    ret i8* %pixels1.sub
+define dso_local i8* @toplevel() {
+entry:
+  %pixels = alloca [500 x %struct.Pixel]
+  call void @llvm.dbg.declare(metadata [500 x %struct.Pixel]* %pixels, metadata !11, metadata !DIExpression()), !dbg !12
+  %arraydecay = bitcast [500 x %struct.Pixel]* %pixels to i8*
+  ret i8* %arraydecay
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 11.1.0", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.cpp", directory: "/path/to/test_cpp")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 11.1.0"}
+!7 = distinct !DISubprogram(name: "toplevel", linkageName: "_Z8toplevelPc", scope: !1, file: !1, line: 9, type: !8, scopeLine: 9, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !2)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10, !10}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !DILocalVariable(name: "num", arg: 1, scope: !7, file: !1, line: 9, type: !10)
+!12 = !DILocation(line: 9, column: 16, scope: !7)
+!13 = !DILocalVariable(name: "Pixel", scope: !7, file: !1, line: 10, type: !14)
+!14 = !DICompositeType(tag: DW_TAG_array_type, baseType: !10, size: 16000, elements: !15)
+!15 = !{!16}
+!16 = !DISubrange(count: 500)
+!17 = !DILocation(line: 10, column: 9, scope: !7)
+!18 = !DILocation(line: 12, column: 5, scope: !7)
diff --git a/llvm/test/Transforms/InstCombine/no-undef-bitcast-to-gep.ll b/llvm/test/Transforms/InstCombine/no-undef-bitcast-to-gep.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/no-undef-bitcast-to-gep.ll
@@ -0,0 +1,48 @@
+; RUN: opt -S -instcombine %s | FileCheck %s
+
+; https://github.com/llvm/llvm-project/issues/56807
+%struct.Pixel = type { i8, i8, i8 }
+
+declare void @foo(i8* %pixels)
+
+declare void @llvm.dbg.declare(metadata, metadata, metadata)
+
+; CHECK-LABEL: @toplevel(
+; CHECK:  entry:
+; CHECK-NEXT:    %pixels1 = alloca [1500 x i8], align 8
+; CHECK-NEXT:    call void @llvm.dbg.declare(metadata [1500 x i8]* %pixels1, metadata !7, metadata !DIExpression()), !dbg !12
+; CHECK-NEXT:    %pixels1.sub = getelementptr inbounds [1500 x i8], [1500 x i8]* %pixels1, i64 0, i64 0
+; CHECK-NEXT:    call void @foo(i8* nonnull %pixels1.sub)
+; CHECK-NEXT:    ret void
+define dso_local void @toplevel() {
+entry:
+  %pixels = alloca [500 x %struct.Pixel]
+  call void @llvm.dbg.declare(metadata [500 x %struct.Pixel]* %pixels, metadata !11, metadata !DIExpression()), !dbg !12
+  %arraydecay = bitcast [500 x %struct.Pixel]* %pixels to i8*
+  call void @foo(i8* %arraydecay)
+  ret void
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 11.1.0", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "test.cpp", directory: "/path/to/test_cpp")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 11.1.0"}
+!7 = distinct !DISubprogram(name: "toplevel", linkageName: "_Z8toplevelv", scope: !1, file: !1, line: 9, type: !8, scopeLine: 9, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !2)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10, !10}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !DILocalVariable(name: "num", arg: 1, scope: !7, file: !1, line: 9, type: !10)
+!12 = !DILocation(line: 9, column: 16, scope: !7)
+!13 = !DILocalVariable(name: "Pixel", scope: !7, file: !1, line: 10, type: !14)
+!14 = !DICompositeType(tag: DW_TAG_array_type, baseType: !10, size: 16000, elements: !15)
+!15 = !{!16}
+!16 = !DISubrange(count: 500)
+!17 = !DILocation(line: 10, column: 9, scope: !7)
+!18 = !DILocation(line: 12, column: 5, scope: !7)
\ No newline at end of file