From 2761414777ab29f1716aa84fa407b5bcca7dbfd1 Mon Sep 17 00:00:00 2001
From: Jake Bailey <5341706+jakebailey@users.noreply.github.com>
Date: Mon, 31 Aug 2026 08:31:52 -0700
Subject: [PATCH] compiler: make composite map hashes order dependent

Use the same multiply-and-XOR combination as the runtime interface hash for generated struct and array key hashes. This avoids systematic collisions when components are permuted.
---
 compiler/map.go              |  8 +++++---
 compiler/testdata/zeromap.ll | 25 ++++++++++++++-----------
 2 files changed, 19 insertions(+), 14 deletions(-)

diff --git a/compiler/map.go b/compiler/map.go
index 3481ae90a..ea8a49f43 100644
--- a/compiler/map.go
+++ b/compiler/map.go
@@ -417,6 +417,7 @@ func (b *builder) generateKeyHash(keyType types.Type, llvmKeyType llvm.Type, key
 		return b.createRuntimeCall("hashmapInterfacePtrHash", []llvm.Value{keyPtr, size, seed}, "hash")
 	case *types.Struct:
 		hash := llvm.ConstInt(b.ctx.Int32Type(), 0, false)
+		multiplier := llvm.ConstInt(b.ctx.Int32Type(), 31, false)
 		zero := llvm.ConstInt(b.ctx.Int32Type(), 0, false)
 		for i := 0; i < keyType.NumFields(); i++ {
 			if keyType.Field(i).Name() == "_" {
@@ -430,7 +431,7 @@ func (b *builder) generateKeyHash(keyType types.Type, llvmKeyType llvm.Type, key
 			idx := llvm.ConstInt(b.ctx.Int32Type(), uint64(i), false)
 			fieldPtr := b.CreateInBoundsGEP(llvmKeyType, keyPtr, []llvm.Value{zero, idx}, "")
 			fieldHash := b.generateKeyHash(fieldType, llvmFieldType, fieldPtr, seed)
-			hash = b.CreateXor(hash, fieldHash, "")
+			hash = b.CreateXor(b.CreateMul(hash, multiplier, ""), fieldHash, "")
 		}
 		return hash
 	case *types.Array:
@@ -445,6 +446,7 @@ func (b *builder) generateKeyHash(keyType types.Type, llvmKeyType llvm.Type, key
 		if arrayLen == 0 {
 			return llvm.ConstInt(b.ctx.Int32Type(), 0, false)
 		}
+		multiplier := llvm.ConstInt(b.ctx.Int32Type(), 31, false)
 		if arrayLen <= hashArrayUnrollLimit {
 			hash := llvm.ConstInt(b.ctx.Int32Type(), 0, false)
 			zero := llvm.ConstInt(b.ctx.Int32Type(), 0, false)
@@ -452,7 +454,7 @@ func (b *builder) generateKeyHash(keyType types.Type, llvmKeyType llvm.Type, key
 				idx := llvm.ConstInt(b.uintptrType, uint64(i), false)
 				elemPtr := b.CreateInBoundsGEP(llvmKeyType, keyPtr, []llvm.Value{zero, idx}, "")
 				elemHash := b.generateKeyHash(elemType, llvmElemType, elemPtr, seed)
-				hash = b.CreateXor(hash, elemHash, "")
+				hash = b.CreateXor(b.CreateMul(hash, multiplier, ""), elemHash, "")
 			}
 			return hash
 		}
@@ -471,7 +473,7 @@ func (b *builder) generateKeyHash(keyType types.Type, llvmKeyType llvm.Type, key
 
 		elemPtr := b.CreateInBoundsGEP(llvmKeyType, keyPtr, []llvm.Value{zero, phiI}, "")
 		elemHash := b.generateKeyHash(elemType, llvmElemType, elemPtr, seed)
-		newHash := b.CreateXor(phiHash, elemHash, "")
+		newHash := b.CreateXor(b.CreateMul(phiHash, multiplier, ""), elemHash, "")
 		nextI := b.CreateAdd(phiI, llvm.ConstInt(b.uintptrType, 1, false), "")
 		cond := b.CreateICmp(llvm.IntULT, nextI, llvm.ConstInt(b.uintptrType, uint64(arrayLen), false), "")
 		b.CreateCondBr(cond, loopBody, loopDone)
diff --git a/compiler/testdata/zeromap.ll b/compiler/testdata/zeromap.ll
index 03f96f2fa..3becf83aa 100644
--- a/compiler/testdata/zeromap.ll
+++ b/compiler/testdata/zeromap.ll
@@ -113,8 +113,9 @@ entry:
   %hash = call i32 @runtime.hashmapStringPtrHash(ptr %0, i32 8, i32 %2, ptr undef) #4
   %4 = getelementptr inbounds nuw i8, ptr %0, i32 8
   %hash1 = call i32 @runtime.hashmapStringPtrHash(ptr nonnull %4, i32 8, i32 %2, ptr undef) #4
-  %5 = xor i32 %hash, %hash1
-  ret i32 %5
+  %5 = mul i32 %hash, 31
+  %6 = xor i32 %5, %hash1
+  ret i32 %6
 }
 
 declare i32 @runtime.hashmapStringPtrHash(ptr, i32, i32, ptr) #0
@@ -161,8 +162,9 @@ entry:
   %hash = call i32 @runtime.hashmapStringPtrHash(ptr %0, i32 8, i32 %2, ptr undef) #4
   %4 = getelementptr inbounds nuw i8, ptr %0, i32 8
   %hash1 = call i32 @runtime.hashmapStringPtrHash(ptr nonnull %4, i32 8, i32 %2, ptr undef) #4
-  %5 = xor i32 %hash, %hash1
-  ret i32 %5
+  %5 = mul i32 %hash, 31
+  %6 = xor i32 %5, %hash1
+  ret i32 %6
 }
 
 ; Function Attrs: nounwind
@@ -203,17 +205,18 @@ entry:
   br label %hash.array.body
 
 hash.array.body:                                  ; preds = %hash.array.body, %entry
-  %i = phi i32 [ 0, %entry ], [ %6, %hash.array.body ]
-  %hash.acc = phi i32 [ 0, %entry ], [ %5, %hash.array.body ]
+  %i = phi i32 [ 0, %entry ], [ %7, %hash.array.body ]
+  %hash.acc = phi i32 [ 0, %entry ], [ %6, %hash.array.body ]
   %4 = getelementptr inbounds nuw %runtime._string, ptr %0, i32 %i
   %hash = call i32 @runtime.hashmapStringPtrHash(ptr %4, i32 8, i32 %2, ptr undef) #4
-  %5 = xor i32 %hash.acc, %hash
-  %6 = add nuw nsw i32 %i, 1
-  %7 = icmp samesign ult i32 %i, 4
-  br i1 %7, label %hash.array.body, label %hash.array.done
+  %5 = mul i32 %hash.acc, 31
+  %6 = xor i32 %5, %hash
+  %7 = add nuw nsw i32 %i, 1
+  %8 = icmp samesign ult i32 %i, 4
+  br i1 %8, label %hash.array.body, label %hash.array.done
 
 hash.array.done:                                  ; preds = %hash.array.body
-  ret i32 %5
+  ret i32 %6
 }
 
 ; Function Attrs: nounwind