transform: avoid byte slice string length allocations

Rewrite len(string(b)) to use the original slice length. This is always
valid even when the copied string must remain for other uses.

Author: jakebailey

transform/rtcalls.go

@@ -69,6 +69,23 @@ func OptimizeStringToBytes(mod llvm.Module) {
 	}
 }
 
+func optimizeStringFromBytesLen(stringFromBytes llvm.Value) {
+	for _, call := range getUses(stringFromBytes) {
+		for _, use := range getUses(call) {
+			if use.IsAExtractValueInst().IsNil() {
+				continue
+			}
+			indices := use.Indices()
+			if len(indices) != 1 || indices[0] != 1 {
+				continue
+			}
+			use.ReplaceAllUsesWith(call.Operand(1))
+			use.EraseFromParentAsInstruction()
+		}
+		removeDeadStringFromBytes(stringFromBytesValue{call: call})
+	}
+}
+
 // OptimizeStringFromBytes transforms temporary strings created from []byte
 // slices into direct uses of the slice data when no instruction between the
 // conversion and use can mutate the slice.
@@ -96,6 +113,7 @@ func OptimizeStringFromBytes(mod llvm.Module) {
 	for _, compare := range compareFuncs {
 		optimizeStringFromBytesCompareUses(stringFromBytes, compare, safeCalls)
 	}
+	optimizeStringFromBytesLen(stringFromBytes)
 }
 
 func optimizeStringFromBytesCompareUses(stringFromBytes, compare llvm.Value, safeCalls map[llvm.Value]struct{}) {

transform/rtcalls_test.go

@@ -40,6 +40,6 @@ func TestOptimizeStringFromBytesStringLess(t *testing.T) {
 func TestOptimizeStringFromBytesLen(t *testing.T) {
 	t.Parallel()
 	testTransform(t, "testdata/stringfrombytes-len", func(mod llvm.Module) {
-		// TODO: optimize the []byte-to-string conversion away.
+		transform.OptimizeStringFromBytes(mod)
 	})
 }

transform/testdata/stringfrombytes-len.out.ll

@@ -12,20 +12,13 @@ declare void @useString(ptr, i32)
 define i32 @main.stringFromBytesLen(ptr %a.data, i32 %a.len, i32 %a.cap, ptr %context) {
 entry:
   %stackalloc = alloca i8, align 1
-  %0 = call %runtime._string @runtime.stringFromBytes(ptr %a.data, i32 %a.len, i32 %a.cap, ptr undef)
-  %1 = extractvalue %runtime._string %0, 0
-  call void @runtime.trackPointer(ptr %1, ptr %stackalloc, ptr undef)
-  %2 = extractvalue %runtime._string %0, 1
-  %len = extractvalue %runtime._string %0, 1
-  ret i32 %len
+  ret i32 %a.len
 }
 
 define i32 @main.keepStringConversion(ptr %a.data, i32 %a.len, i32 %a.cap, ptr %context) {
 entry:
   %0 = call %runtime._string @runtime.stringFromBytes(ptr %a.data, i32 %a.len, i32 %a.cap, ptr undef)
   %1 = extractvalue %runtime._string %0, 0
-  %2 = extractvalue %runtime._string %0, 1
-  call void @useString(ptr %1, i32 %2)
-  %len = extractvalue %runtime._string %0, 1
-  ret i32 %len
+  call void @useString(ptr %1, i32 %a.len)
+  ret i32 %a.len
 }

transform/testdata/stringfrombytes-stringequal.out.ll

@@ -31,31 +31,25 @@ define i1 @main.keepStringConversion(ptr %a.data, i32 %a.len, i32 %a.cap, ptr %b
 entry:
   %0 = call %runtime._string @runtime.stringFromBytes(ptr %a.data, i32 %a.len, i32 %a.cap, ptr undef)
   %1 = extractvalue %runtime._string %0, 0
-  %2 = extractvalue %runtime._string %0, 1
-  call void @useString(ptr %1, i32 %2)
-  %3 = call i1 @runtime.stringEqual(ptr %1, i32 %2, ptr %b.data, i32 %b.len, ptr undef)
-  ret i1 %3
+  call void @useString(ptr %1, i32 %a.len)
+  %2 = call i1 @runtime.stringEqual(ptr %1, i32 %a.len, ptr %b.data, i32 %b.len, ptr undef)
+  ret i1 %2
 }
 
 define i32 @main.equalAndLen(ptr %a.data, i32 %a.len, i32 %a.cap, ptr %s.data, i32 %s.len, ptr %context) {
 entry:
-  %0 = call %runtime._string @runtime.stringFromBytes(ptr %a.data, i32 %a.len, i32 %a.cap, ptr undef)
-  %1 = extractvalue %runtime._string %0, 0
-  %2 = extractvalue %runtime._string %0, 1
   %equal = call i1 @runtime.stringEqual(ptr %a.data, i32 %a.len, ptr %s.data, i32 %s.len, ptr undef)
-  %len = extractvalue %runtime._string %0, 1
   %equal.ext = zext i1 %equal to i32
-  %result = add i32 %len, %equal.ext
+  %result = add i32 %a.len, %equal.ext
   ret i32 %result
 }
 
 define i1 @main.equalBeforeOtherUse(ptr %a.data, i32 %a.len, i32 %a.cap, ptr %s.data, i32 %s.len, ptr %context) {
 entry:
   %0 = call %runtime._string @runtime.stringFromBytes(ptr %a.data, i32 %a.len, i32 %a.cap, ptr undef)
   %1 = extractvalue %runtime._string %0, 0
-  %2 = extractvalue %runtime._string %0, 1
   %equal = call i1 @runtime.stringEqual(ptr %a.data, i32 %a.len, ptr %s.data, i32 %s.len, ptr undef)
-  call void @useString(ptr %1, i32 %2)
+  call void @useString(ptr %1, i32 %a.len)
   ret i1 %equal
 }
 
@@ -71,10 +65,9 @@ define i1 @main.keepComparisonAfterMutation(ptr %a.data, i32 %a.len, i32 %a.cap,
 entry:
   %0 = call %runtime._string @runtime.stringFromBytes(ptr %a.data, i32 %a.len, i32 %a.cap, ptr undef)
   %1 = extractvalue %runtime._string %0, 0
-  %2 = extractvalue %runtime._string %0, 1
   %equal1 = call i1 @runtime.stringEqual(ptr %a.data, i32 %a.len, ptr %s.data, i32 %s.len, ptr undef)
   store i8 1, ptr %a.data, align 1
-  %equal2 = call i1 @runtime.stringEqual(ptr %1, i32 %2, ptr %s.data, i32 %s.len, ptr undef)
+  %equal2 = call i1 @runtime.stringEqual(ptr %1, i32 %a.len, ptr %s.data, i32 %s.len, ptr undef)
   %result = and i1 %equal1, %equal2
   ret i1 %result
 }
@@ -84,9 +77,8 @@ entry:
   %a.data = alloca [4 x i8], align 1
   %0 = call %runtime._string @runtime.stringFromBytes(ptr %a.data, i32 4, i32 4, ptr undef)
   %1 = extractvalue %runtime._string %0, 0
-  %2 = extractvalue %runtime._string %0, 1
   call void @llvm.lifetime.end.p0(ptr %a.data)
-  %equal = call i1 @runtime.stringEqual(ptr %1, i32 %2, ptr %s.data, i32 %s.len, ptr undef)
+  %equal = call i1 @runtime.stringEqual(ptr %1, i32 4, ptr %s.data, i32 %s.len, ptr undef)
   ret i1 %equal
 }
 

transform/testdata/stringfrombytes-stringless.out.ll

@@ -34,8 +34,7 @@ define i1 @main.keepStringConversion(ptr %a.data, i32 %a.len, i32 %a.cap, ptr %b
 entry:
   %0 = call %runtime._string @runtime.stringFromBytes(ptr %a.data, i32 %a.len, i32 %a.cap, ptr undef)
   %1 = extractvalue %runtime._string %0, 0
-  %2 = extractvalue %runtime._string %0, 1
-  call void @useString(ptr %1, i32 %2)
-  %3 = call i1 @runtime.stringLess(ptr %1, i32 %2, ptr %b.data, i32 %b.len, ptr undef)
-  ret i1 %3
+  call void @useString(ptr %1, i32 %a.len)
+  %2 = call i1 @runtime.stringLess(ptr %1, i32 %a.len, ptr %b.data, i32 %b.len, ptr undef)
+  ret i1 %2
 }