[llvm] [ADT,CodeGen] Remove stable_hash_combine_string (PR #100668)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-adt

Author: Fangrui Song (MaskRay)

<details>
<summary>Changes</summary>

FNV, used by stable_hash_combine_string is extremely slow. For string
hashing with good avalanche effects, we prefer xxh3_64bits.

StableHashing.h might still be useful as Hashing.h hashes might be
non-deterministic.


---
Full diff: https://github.com/llvm/llvm-project/pull/100668.diff


2 Files Affected:

- (modified) llvm/include/llvm/ADT/StableHashing.h (-12) 
- (modified) llvm/lib/CodeGen/MachineStableHash.cpp (+4-4) 


``````````diff
diff --git a/llvm/include/llvm/ADT/StableHashing.h b/llvm/include/llvm/ADT/StableHashing.h
index 884b5752d9bb0..f675f828f702e 100644
--- a/llvm/include/llvm/ADT/StableHashing.h
+++ b/llvm/include/llvm/ADT/StableHashing.h
@@ -95,18 +95,6 @@ inline stable_hash stable_hash_combine_array(const stable_hash *P, size_t C) {
     hashing::detail::stable_hash_append(Hash, P[I]);
   return Hash;
 }
-
-inline stable_hash stable_hash_combine_string(const StringRef &S) {
-  return stable_hash_combine_range(S.begin(), S.end());
-}
-
-inline stable_hash stable_hash_combine_string(const char *C) {
-  stable_hash Hash = hashing::detail::FNV_OFFSET_64;
-  while (*C)
-    hashing::detail::stable_hash_append(Hash, *(C++));
-  return Hash;
-}
-
 } // namespace llvm
 
 #endif
diff --git a/llvm/lib/CodeGen/MachineStableHash.cpp b/llvm/lib/CodeGen/MachineStableHash.cpp
index 5abfbd5981fba..d2e02a2d739c1 100644
--- a/llvm/lib/CodeGen/MachineStableHash.cpp
+++ b/llvm/lib/CodeGen/MachineStableHash.cpp
@@ -33,6 +33,7 @@
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/Support/Alignment.h"
 #include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/xxhash.h"
 
 #define DEBUG_TYPE "machine-stable-hash"
 
@@ -100,8 +101,7 @@ stable_hash llvm::stableHashValue(const MachineOperand &MO) {
   case MachineOperand::MO_TargetIndex: {
     if (const char *Name = MO.getTargetIndexName())
       return stable_hash_combine(MO.getType(), MO.getTargetFlags(),
-                                 stable_hash_combine_string(Name),
-                                 MO.getOffset());
+                                 xxh3_64bits(Name), MO.getOffset());
     StableHashBailingTargetIndexNoName++;
     return 0;
   }
@@ -113,7 +113,7 @@ stable_hash llvm::stableHashValue(const MachineOperand &MO) {
 
   case MachineOperand::MO_ExternalSymbol:
     return hash_combine(MO.getType(), MO.getTargetFlags(), MO.getOffset(),
-                        stable_hash_combine_string(MO.getSymbolName()));
+                        xxh3_64bits(MO.getSymbolName()));
 
   case MachineOperand::MO_RegisterMask:
   case MachineOperand::MO_RegisterLiveOut: {
@@ -151,7 +151,7 @@ stable_hash llvm::stableHashValue(const MachineOperand &MO) {
   case MachineOperand::MO_MCSymbol: {
     auto SymbolName = MO.getMCSymbol()->getName();
     return hash_combine(MO.getType(), MO.getTargetFlags(),
-                        stable_hash_combine_string(SymbolName));
+                        xxh3_64bits(SymbolName));
   }
   case MachineOperand::MO_CFIIndex:
     return stable_hash_combine(MO.getType(), MO.getTargetFlags(),

``````````

</details>


https://github.com/llvm/llvm-project/pull/100668