BPF: support inlining __builtin_memcmp intrinsic call

Delyan Kratunov reported an issue where __builtin_memcmp is
not inlined into simple load/compare instructions.
This is a known issue. In the current state, __builtin_memcmp
will be converted to memcmp call which won't work for
bpf programs.

This patch added support for expanding __builtin_memcmp with
actual loads and compares up to currently maximum 128 total loads.
The implementation is identical to PowerPC.

Differential Revision: https://reviews.llvm.org/D122676

Author: yonghong-song

llvm/lib/Target/BPF/BPFISelLowering.cpp

@@ -168,6 +168,7 @@ BPFTargetLowering::BPFTargetLowering(const TargetMachine &TM,
     MaxStoresPerMemset = MaxStoresPerMemsetOptSize = 0;
     MaxStoresPerMemcpy = MaxStoresPerMemcpyOptSize = 0;
     MaxStoresPerMemmove = MaxStoresPerMemmoveOptSize = 0;
+    MaxLoadsPerMemcmp = 0;
   } else {
     // inline memcpy() for kernel to see explicit copy
     unsigned CommonMaxStores =
@@ -176,6 +177,7 @@ BPFTargetLowering::BPFTargetLowering(const TargetMachine &TM,
     MaxStoresPerMemset = MaxStoresPerMemsetOptSize = CommonMaxStores;
     MaxStoresPerMemcpy = MaxStoresPerMemcpyOptSize = CommonMaxStores;
     MaxStoresPerMemmove = MaxStoresPerMemmoveOptSize = CommonMaxStores;
+    MaxLoadsPerMemcmp = MaxLoadsPerMemcmpOptSize = CommonMaxStores;
   }
 
   // CPU/Feature control

llvm/lib/Target/BPF/BPFTargetTransformInfo.h

@@ -71,6 +71,15 @@ class BPFTTIImpl : public BasicTTIImplBase<BPFTTIImpl> {
                                            Opd2Info, Opd1PropInfo,
                                            Opd2PropInfo);
   }
+
+  TTI::MemCmpExpansionOptions enableMemCmpExpansion(bool OptSize,
+                                                    bool IsZeroCmp) const {
+    TTI::MemCmpExpansionOptions Options;
+    Options.LoadSizes = {8, 4, 2, 1};
+    Options.MaxNumLoads = TLI->getMaxExpandSizeMemcmp(OptSize);
+    return Options;
+  }
+
 };
 
 } // end namespace llvm

llvm/test/CodeGen/BPF/memcmp.ll

@@ -0,0 +1,72 @@
+; RUN: llc -march=bpfel < %s | FileCheck %s
+; RUN: llc -march=bpfel -mcpu=v3 < %s | FileCheck %s
+;
+; Source code:
+;   /* set aligned 4 to minimize the number of loads */
+;   struct build_id {
+;     unsigned char id[20];
+;   } __attribute__((aligned(4)));
+;
+;   /* try to compute a local build_id */
+;   void bar1(void *);
+;
+;   /* the global build_id to compare */
+;   struct build_id id2;
+;
+;   int foo()
+;   {
+;     struct build_id id1;
+;
+;     bar1(&id1);
+;     return __builtin_memcmp(&id1, &id2, sizeof(id1)) == 0;
+;   }
+; Compilation flags:
+;   clang -target bpf -S -O2 t.c -emit-llvm
+
+
+%struct.build_id = type { [20 x i8] }
+
+@id2 = dso_local global %struct.build_id zeroinitializer, align 4
+
+; Function Attrs: nounwind
+define dso_local i32 @foo() local_unnamed_addr #0 {
+entry:
+  %id11 = alloca [20 x i8], align 4
+  %id11.sub = getelementptr inbounds [20 x i8], [20 x i8]* %id11, i64 0, i64 0
+  call void @llvm.lifetime.start.p0i8(i64 20, i8* nonnull %id11.sub) #4
+  call void @bar1(i8* noundef nonnull %id11.sub) #4
+  %call = call i32 @memcmp(i8* noundef nonnull dereferenceable(20) %id11.sub, i8* noundef nonnull dereferenceable(20) getelementptr inbounds (%struct.build_id, %struct.build_id* @id2, i64 0, i32 0, i64 0), i64 noundef 20) #4
+  %cmp = icmp eq i32 %call, 0
+  %conv = zext i1 %cmp to i32
+  call void @llvm.lifetime.end.p0i8(i64 20, i8* nonnull %id11.sub) #4
+  ret i32 %conv
+}
+
+; CHECK:   *(u32 *)(r1 + 0)
+; CHECK:   *(u32 *)(r10 - 20)
+; CHECK:   *(u32 *)(r10 - 12)
+; CHECK:   *(u32 *)(r1 + 8)
+
+; Function Attrs: argmemonly mustprogress nofree nosync nounwind willreturn
+declare void @llvm.lifetime.start.p0i8(i64 immarg, i8* nocapture) #1
+
+declare dso_local void @bar1(i8* noundef) local_unnamed_addr #2
+
+; Function Attrs: argmemonly mustprogress nofree nounwind readonly willreturn
+declare dso_local i32 @memcmp(i8* nocapture noundef, i8* nocapture noundef, i64 noundef) local_unnamed_addr #3
+
+; Function Attrs: argmemonly mustprogress nofree nosync nounwind willreturn
+declare void @llvm.lifetime.end.p0i8(i64 immarg, i8* nocapture) #1
+
+attributes #0 = { nounwind "frame-pointer"="all" "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
+attributes #1 = { argmemonly mustprogress nofree nosync nounwind willreturn }
+attributes #2 = { "frame-pointer"="all" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
+attributes #3 = { argmemonly mustprogress nofree nounwind readonly willreturn "frame-pointer"="all" "no-trapping-math"="true" "stack-protector-buffer-size"="8" }
+attributes #4 = { nounwind }
+
+!llvm.module.flags = !{!0, !1}
+!llvm.ident = !{!2}
+
+!0 = !{i32 1, !"wchar_size", i32 4}
+!1 = !{i32 7, !"frame-pointer", i32 2}
+!2 = !{!"clang version 15.0.0 (https://github.com/llvm/llvm-project.git dea65874b2505f8f5e8e51fd8cad6908feb375ec)"}