compiler: mark string parameters as readonly

Strings are readonly, but the compiler doesn't always know this. Marking
them as readonly in the frontend allows the compiler to optimize based
on this knowledge.

This provides some small code size benefits. I didn't measure running
speed.

compiler/calls.go

@@ -32,6 +32,9 @@ const (
 	// Whether this is a full or partial Go parameter (int, slice, etc).
 	// The extra context parameter is not a Go parameter.
 	paramIsGoParam = 1 << iota
+
+	// Whether this is a readonly parameter (for example, a string pointer).
+	paramIsReadonly
 )
 
 // createRuntimeCallCommon creates a runtime call. Use createRuntimeCall or
@@ -167,6 +170,7 @@ func (c *compilerContext) flattenAggregateType(t llvm.Type, name string, goType
 				continue
 			}
 			suffix := strconv.Itoa(i)
+			isString := false
 			if goType != nil {
 				// Try to come up with a good suffix for this struct field,
 				// depending on which Go type it's based on.
@@ -183,12 +187,16 @@ func (c *compilerContext) flattenAggregateType(t llvm.Type, name string, goType
 						suffix = []string{"r", "i"}[i]
 					case types.String:
 						suffix = []string{"data", "len"}[i]
+						isString = true
 					}
 				case *types.Signature:
 					suffix = []string{"context", "funcptr"}[i]
 				}
 			}
 			subInfos := c.flattenAggregateType(subfield, name+"."+suffix, extractSubfield(goType, i))
+			if isString {
+				subInfos[0].flags |= paramIsReadonly
+			}
 			paramInfos = append(paramInfos, subInfos...)
 		}
 		return paramInfos

compiler/symbol.go

@@ -142,6 +142,11 @@ func (c *compilerContext) getFunction(fn *ssa.Function) (llvm.Type, llvm.Value)
 			nocapture := c.ctx.CreateEnumAttribute(llvm.AttributeKindID("nocapture"), 0)
 			llvmFn.AddAttributeAtIndex(i+1, nocapture)
 		}
+		if paramInfo.flags&paramIsReadonly != 0 && paramInfo.llvmType.TypeKind() == llvm.PointerTypeKind {
+			// Readonly pointer parameters (like strings) benefit from being marked as readonly.
+			readonly := c.ctx.CreateEnumAttribute(llvm.AttributeKindID("readonly"), 0)
+			llvmFn.AddAttributeAtIndex(i+1, readonly)
+		}
 	}
 
 	// Set a number of function or parameter attributes, depending on the

compiler/testdata/go1.20.ll

@@ -50,7 +50,7 @@ unsafe.String.throw:                              ; preds = %entry
 declare void @runtime.unsafeSlicePanic(ptr) #1
 
 ; Function Attrs: nounwind
-define hidden ptr @main.unsafeStringData(ptr %s.data, i32 %s.len, ptr %context) unnamed_addr #2 {
+define hidden ptr @main.unsafeStringData(ptr readonly %s.data, i32 %s.len, ptr %context) unnamed_addr #2 {
 entry:
   %stackalloc = alloca i8, align 1
   call void @runtime.trackPointer(ptr %s.data, ptr nonnull %stackalloc, ptr undef) #3

compiler/testdata/go1.21.ll

@@ -77,7 +77,7 @@ entry:
 }
 
 ; Function Attrs: nounwind
-define hidden %runtime._string @main.minString(ptr %a.data, i32 %a.len, ptr %b.data, i32 %b.len, ptr %context) unnamed_addr #2 {
+define hidden %runtime._string @main.minString(ptr readonly %a.data, i32 %a.len, ptr readonly %b.data, i32 %b.len, ptr %context) unnamed_addr #2 {
 entry:
   %0 = insertvalue %runtime._string zeroinitializer, ptr %a.data, 0
   %1 = insertvalue %runtime._string %0, i32 %a.len, 1
@@ -91,7 +91,7 @@ entry:
   ret %runtime._string %5
 }
 
-declare i1 @runtime.stringLess(ptr, i32, ptr, i32, ptr) #1
+declare i1 @runtime.stringLess(ptr readonly, i32, ptr readonly, i32, ptr) #1
 
 ; Function Attrs: nounwind
 define hidden i32 @main.maxInt(i32 %a, i32 %b, ptr %context) unnamed_addr #2 {
@@ -116,7 +116,7 @@ entry:
 }
 
 ; Function Attrs: nounwind
-define hidden %runtime._string @main.maxString(ptr %a.data, i32 %a.len, ptr %b.data, i32 %b.len, ptr %context) unnamed_addr #2 {
+define hidden %runtime._string @main.maxString(ptr readonly %a.data, i32 %a.len, ptr readonly %b.data, i32 %b.len, ptr %context) unnamed_addr #2 {
 entry:
   %0 = insertvalue %runtime._string zeroinitializer, ptr %a.data, 0
   %1 = insertvalue %runtime._string %0, i32 %a.len, 1

compiler/testdata/string.ll

@@ -31,13 +31,13 @@ entry:
 }
 
 ; Function Attrs: nounwind
-define hidden i32 @main.stringLen(ptr %s.data, i32 %s.len, ptr %context) unnamed_addr #2 {
+define hidden i32 @main.stringLen(ptr readonly %s.data, i32 %s.len, ptr %context) unnamed_addr #2 {
 entry:
   ret i32 %s.len
 }
 
 ; Function Attrs: nounwind
-define hidden i8 @main.stringIndex(ptr %s.data, i32 %s.len, i32 %index, ptr %context) unnamed_addr #2 {
+define hidden i8 @main.stringIndex(ptr readonly %s.data, i32 %s.len, i32 %index, ptr %context) unnamed_addr #2 {
 entry:
   %.not = icmp ult i32 %index, %s.len
   br i1 %.not, label %lookup.next, label %lookup.throw
@@ -55,16 +55,16 @@ lookup.throw:                                     ; preds = %entry
 declare void @runtime.lookupPanic(ptr) #1
 
 ; Function Attrs: nounwind
-define hidden i1 @main.stringCompareEqual(ptr %s1.data, i32 %s1.len, ptr %s2.data, i32 %s2.len, ptr %context) unnamed_addr #2 {
+define hidden i1 @main.stringCompareEqual(ptr readonly %s1.data, i32 %s1.len, ptr readonly %s2.data, i32 %s2.len, ptr %context) unnamed_addr #2 {
 entry:
   %0 = call i1 @runtime.stringEqual(ptr %s1.data, i32 %s1.len, ptr %s2.data, i32 %s2.len, ptr undef) #3
   ret i1 %0
 }
 
-declare i1 @runtime.stringEqual(ptr, i32, ptr, i32, ptr) #1
+declare i1 @runtime.stringEqual(ptr readonly, i32, ptr readonly, i32, ptr) #1
 
 ; Function Attrs: nounwind
-define hidden i1 @main.stringCompareUnequal(ptr %s1.data, i32 %s1.len, ptr %s2.data, i32 %s2.len, ptr %context) unnamed_addr #2 {
+define hidden i1 @main.stringCompareUnequal(ptr readonly %s1.data, i32 %s1.len, ptr readonly %s2.data, i32 %s2.len, ptr %context) unnamed_addr #2 {
 entry:
   %0 = call i1 @runtime.stringEqual(ptr %s1.data, i32 %s1.len, ptr %s2.data, i32 %s2.len, ptr undef) #3
   %1 = xor i1 %0, true
@@ -72,16 +72,16 @@ entry:
 }
 
 ; Function Attrs: nounwind
-define hidden i1 @main.stringCompareLarger(ptr %s1.data, i32 %s1.len, ptr %s2.data, i32 %s2.len, ptr %context) unnamed_addr #2 {
+define hidden i1 @main.stringCompareLarger(ptr readonly %s1.data, i32 %s1.len, ptr readonly %s2.data, i32 %s2.len, ptr %context) unnamed_addr #2 {
 entry:
   %0 = call i1 @runtime.stringLess(ptr %s2.data, i32 %s2.len, ptr %s1.data, i32 %s1.len, ptr undef) #3
   ret i1 %0
 }
 
-declare i1 @runtime.stringLess(ptr, i32, ptr, i32, ptr) #1
+declare i1 @runtime.stringLess(ptr readonly, i32, ptr readonly, i32, ptr) #1
 
 ; Function Attrs: nounwind
-define hidden i8 @main.stringLookup(ptr %s.data, i32 %s.len, i8 %x, ptr %context) unnamed_addr #2 {
+define hidden i8 @main.stringLookup(ptr readonly %s.data, i32 %s.len, i8 %x, ptr %context) unnamed_addr #2 {
 entry:
   %0 = zext i8 %x to i32
   %.not = icmp ugt i32 %s.len, %0