internal/analysis: don't rewrite loops whose names the new code would capture

.github/workflows/test.yml

@@ -61,8 +61,13 @@ jobs:
       - name: Test tsvc (amd64, SIMD)
         run: make tsvc-test
 
-      - name: Install qemu-user-static
-        run: sudo apt-get update && sudo apt-get install -y qemu-user-static
+      # ubuntu-26.04 has no qemu-user-static: qemu-user ships the static
+      # emulators there, without the -static names the Makefile runs.
+      - name: Install qemu-user
+        run: |
+          sudo apt-get update && sudo apt-get install -y qemu-user
+          command -v qemu-aarch64-static || sudo ln -s "$(command -v qemu-aarch64)" /usr/local/bin/qemu-aarch64-static
+          qemu-aarch64-static --version
 
       - name: Test tsvc (arm64, qemu)
         run: make tsvc-test-qemu-arm64

BUGS.md

@@ -2,11 +2,11 @@
 
 loopvec rewrites code to use `GOEXPERIMENT=simd`, which is new, so most of what it hits
 is in the Go toolchain. This file lists the Go issues that affect loopvec. "tip" means
-gotip, "1.27.1" is the release toolchain loopvec is tested against.
+gotip, "1.27.3" is the release toolchain loopvec is tested against.
 
 | Issue | Status | Affects | loopvec's answer |
 |---|---|---|---|
-| [#80657](https://github.com/golang/go/issues/80657) simd in a method: internal compiler error | fixed on tip (CL 839405), still crashes on 1.27.1 | `-methods` | methods are skipped unless `-methods` is passed |
+| [#80657](https://github.com/golang/go/issues/80657) simd in a method: internal compiler error | fixed on tip (CL 839405), still crashes on 1.27.3 | `-methods` | methods are skipped unless `-methods` is passed |
 | [#80835](https://github.com/golang/go/issues/80835) legacy SSE encodings in functions using simd cause AVX-SSE transition penalties | open, fix pending (CLs 825145, 825146, in tip) | speed of any scalar float code that runs after a rewritten loop, in the same function or another | none; the portable `simd` package has no `ClearAVXUpperBits` |
 | [#81844](https://github.com/golang/go/issues/81844) blank identifier parameters rejected | fixed on tip (CL 841145) | every rewrite of a function with a `_` parameter | the generated file renames them `_p0`, `_p1`, ... |
 | [#81845](https://github.com/golang/go/issues/81845) calls left unresolved when inlined into a package initializer | closed as a duplicate of [#80689](https://github.com/golang/go/issues/80689), which is open | loops in package-level `var x = func() {...}()` | not rewritten (the generated initializer fails to link), see `package_initializer_no_rewrite.txt` and `toolexec_package_initializer.txt` |
@@ -19,7 +19,7 @@ gotip, "1.27.1" is the release toolchain loopvec is tested against.
 
 A function with a receiver that contains simd code failed to compile with
 `internal compiler error: missing Types entry`. loopvec does not rewrite loops in methods
-by default; `-methods` turns it on. The fix is on tip. On 1.27.1 the crash is still there,
+by default; `-methods` turns it on. The fix is on tip. On 1.27.3 the crash is still there,
 and `flag_methods_ice.txt` pins it, so that test starts failing when the pinned toolchain
 gets the fix. Reported by someone else; the fix is [CL 839405](https://go.dev/cl/839405).
 

Makefile

@@ -1,4 +1,4 @@
-export GOTOOLCHAIN ?= go1.27.1
+export GOTOOLCHAIN ?= go1.27.3
 
 .DEFAULT_GOAL := build
 

cmd/loopvec-toolexec/compile.go

@@ -321,6 +321,7 @@ func check(fset *token.FileSet, files []*ast.File, c compileArgs, cfg *importcfg
 		Defs:       make(map[*ast.Ident]types.Object),
 		Uses:       make(map[*ast.Ident]types.Object),
 		Selections: make(map[*ast.SelectorExpr]*types.Selection),
+		Scopes:     make(map[ast.Node]*types.Scope),
 	}
 	var errs []error
 	goarch := os.Getenv("GOARCH")

go.mod

@@ -3,7 +3,7 @@ module github.com/mauri870/loopvec
 go 1.27.1
 
 require (
-	golang.org/x/tools v0.51.0
+	golang.org/x/tools v0.52.1-0.20261030190145-0f433775b36d
 	rsc.io/script v0.0.2
 )
 
@@ -207,10 +207,10 @@ require (
 	go.uber.org/zap v1.27.0 // indirect
 	go.yaml.in/yaml/v3 v3.0.5 // indirect
 	golang.org/x/exp/typeparams v0.0.0-20260811152304-ee035b5b010f // indirect
-	golang.org/x/mod v0.41.0 // indirect
-	golang.org/x/sync v0.23.0 // indirect
-	golang.org/x/sys v0.48.0 // indirect
-	golang.org/x/telemetry v0.0.0-20260924152758-ed294f943157 // indirect
+	golang.org/x/mod v0.42.0 // indirect
+	golang.org/x/sync v0.24.0 // indirect
+	golang.org/x/sys v0.49.0 // indirect
+	golang.org/x/telemetry v0.0.0-20261009151841-97e8ea77aa2a // indirect
 	golang.org/x/text v0.41.0 // indirect
 	google.golang.org/protobuf v1.36.12 // indirect
 	gopkg.in/ini.v1 v1.67.0 // indirect

go.sum

@@ -688,8 +688,8 @@ golang.org/x/mod v0.4.1/go.mod h1:s0Qsj1ACt9ePp/hMypM3fl4fZqREWJwdYDEqhRiZZUA=
 golang.org/x/mod v0.6.0-dev.0.20220106191415-9b9b3d81d5e3/go.mod h1:3p9vT2HGsQu2K1YbXdKPJLVgG5VJdoTa1poYQBtP1AY=
 golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4/go.mod h1:jJ57K6gSWd91VN4djpZkiMVwK6gcyfeH4XE8wZrZaV4=
 golang.org/x/mod v0.8.0/go.mod h1:iBbtSCu2XBx23ZKBPSOrRkjjQPZFPuis4dIYUhu/chs=
-golang.org/x/mod v0.41.0 h1:qJmnOUb4YB+FsEuM3HcWucdZASCPGhsX6uljO6pog0c=
-golang.org/x/mod v0.41.0/go.mod h1:Ek9pY8RKWXwsWvd3rQiHYtMqkjSUV+s1Rj7j4H5Ur6o=
+golang.org/x/mod v0.42.0 h1:ICeL3t2L30ljSPHHxDnR4fqVcBt/zR2zJAb9juavmQg=
+golang.org/x/mod v0.42.0/go.mod h1:u5fzIrRInkL8MaO1cbRkUHT64v303YIT5KEqpNr7jRs=
 golang.org/x/net v0.0.0-20180724234803-3673e40ba225/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
 golang.org/x/net v0.0.0-20180826012351-8a410e7b638d/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
 golang.org/x/net v0.0.0-20181114220301-adae6a3d119a/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
@@ -724,8 +724,8 @@ golang.org/x/net v0.0.0-20210525063256-abc453219eb5/go.mod h1:9nx3DQGgdP8bBQD5qx
 golang.org/x/net v0.0.0-20211015210444-4f30a5c0130f/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
 golang.org/x/net v0.0.0-20220722155237-a158d28d115b/go.mod h1:XRhObCWvk6IyKnWLug+ECip1KBveYUHfp+8e9klMJ9c=
 golang.org/x/net v0.6.0/go.mod h1:2Tu9+aMcznHK/AK1HMvgo6xiTLG5rD5rZLDS+rp2Bjs=
-golang.org/x/net v0.59.0 h1:5zfYln+w5XCxwrnMMJPufRgNoXEaGxl0wo5GqPXyues=
-golang.org/x/net v0.59.0/go.mod h1:2DA/G1UfVbCpQPeWTmMPGY7Cs2PkBkwu743bVX5PIVg=
+golang.org/x/net v0.61.0 h1:RR+6j/BTBCrggOLugxa+66R8FvoN6MHlzDlZIbN9YFw=
+golang.org/x/net v0.61.0/go.mod h1:WgDCOTH8iwtB66T4/GfI94AHo/hhdb7Hah9NDseQfiI=
 golang.org/x/oauth2 v0.0.0-20180821212333-d2e6202438be/go.mod h1:N/0e6XlmueqKjAGxoOufVs8QHGRruUQn6yWY3a++T0U=
 golang.org/x/oauth2 v0.0.0-20190226205417-e64efc72b421/go.mod h1:gOpvHmFTYa4IltrdGE7lF6nIHvwfUNPOp7c8zoXwtLw=
 golang.org/x/oauth2 v0.0.0-20190604053449-0f29369cfe45/go.mod h1:gOpvHmFTYa4IltrdGE7lF6nIHvwfUNPOp7c8zoXwtLw=
@@ -745,8 +745,8 @@ golang.org/x/sync v0.0.0-20201207232520-09787c993a3a/go.mod h1:RxMgew5VJxzue5/jJ
 golang.org/x/sync v0.0.0-20210220032951-036812b2e83c/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.0.0-20220722155255-886fb9371eb4/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
 golang.org/x/sync v0.1.0/go.mod h1:RxMgew5VJxzue5/jJTE5uejpjVlOe/izrB70Jof72aM=
-golang.org/x/sync v0.23.0 h1:KameEIfc1IkluZyXWLn39Wd4tURc6GbCiISGiZm2bQk=
-golang.org/x/sync v0.23.0/go.mod h1:sUUOizhqBxiL6pEWpqNLUiaJn1ShEbZ6BBqskPbjZm0=
+golang.org/x/sync v0.24.0 h1:iqiDrGoDgyuvAKrhLQthUEzU/R378N78gXbBjxed7oA=
+golang.org/x/sync v0.24.0/go.mod h1:sUUOizhqBxiL6pEWpqNLUiaJn1ShEbZ6BBqskPbjZm0=
 golang.org/x/sys v0.0.0-20180830151530-49385e6e1522/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20180905080454-ebe1bf3edb33/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/sys v0.0.0-20181116152217-5ac8a444bdc5/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
@@ -790,10 +790,10 @@ golang.org/x/sys v0.0.0-20220114195835-da31bd327af9/go.mod h1:oPkhp1MJrh7nUepCBc
 golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.0.0-20220722155257-8c9f86f7a55f/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
 golang.org/x/sys v0.5.0/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=
-golang.org/x/sys v0.48.0 h1:bbX/i/6MgT9BVLM9RT1thmxL04yeTAhbEz4SyadbXoo=
-golang.org/x/sys v0.48.0/go.mod h1:hNLxWAXmnKAxqDtdwIYC4bM9oQPEecfsnNMuSxOs3og=
-golang.org/x/telemetry v0.0.0-20260924152758-ed294f943157 h1:DftxASHKl6JolFPqoN+2WrFO53JvkHwRXvFqf8gsivA=
-golang.org/x/telemetry v0.0.0-20260924152758-ed294f943157/go.mod h1:i+ivNqjDnTF3WTElsdk5g9V5DTSBYgdNo7xTU9SDwYA=
+golang.org/x/sys v0.49.0 h1:XbzkgYJHdqh/8m2Uu0W/dQv8nktxx4BFHp1M0gROTXA=
+golang.org/x/sys v0.49.0/go.mod h1:hNLxWAXmnKAxqDtdwIYC4bM9oQPEecfsnNMuSxOs3og=
+golang.org/x/telemetry v0.0.0-20261009151841-97e8ea77aa2a h1:wnveMBKHwx8/nIb8WYT5P9vtefdXOvIWxzycCkc5n6Q=
+golang.org/x/telemetry v0.0.0-20261009151841-97e8ea77aa2a/go.mod h1:i+ivNqjDnTF3WTElsdk5g9V5DTSBYgdNo7xTU9SDwYA=
 golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
 golang.org/x/term v0.0.0-20210927222741-03fcf44c2211/go.mod h1:jbD1KX2456YbFQfuXm/mYQcufACuNUgVhRMnK/tPxf8=
 golang.org/x/term v0.5.0/go.mod h1:jMB1sMXY+tzblOD4FWmEbocvup2/aLOaQEp7JmGp78k=
@@ -857,8 +857,8 @@ golang.org/x/tools v0.1.1-0.20210302220138-2ac05c832e1a/go.mod h1:9bzcO0MWcOuT0t
 golang.org/x/tools v0.1.10/go.mod h1:Uh6Zz+xoGYZom868N8YTex3t7RhtHDBrE8Gzo9bV56E=
 golang.org/x/tools v0.1.12/go.mod h1:hNGJHUnrk76NpqgfD5Aqm5Crs+Hm0VOH/i9J2+nxYbc=
 golang.org/x/tools v0.6.0/go.mod h1:Xwgl3UAJ/d3gWutnCtw505GrjyAbvKui8lOU390QaIU=
-golang.org/x/tools v0.51.0 h1:k4Xc/1Om9jwkBJBo4NVLMSARBoWtK10mx+W5BnXCeAI=
-golang.org/x/tools v0.51.0/go.mod h1:9eEncMayCV6zRMGhR5eZEC2iBx98qWcF1HZ9Z7wJOoA=
+golang.org/x/tools v0.52.1-0.20261030190145-0f433775b36d h1:F5ot9G6RM6pgx3TTdKF6IZa2nocit4tcsrVQlq4Zh5w=
+golang.org/x/tools v0.52.1-0.20261030190145-0f433775b36d/go.mod h1:ItcP5UNaVGG+Yc4eIYmaFLNJ4soEopkx5Y2d4lcEurs=
 golang.org/x/tools/go/expect v0.1.1-deprecated h1:jpBZDwmgPhXsKZC6WhL20P4b/wmnpsEAGHaNy0n/rJM=
 golang.org/x/tools/go/expect v0.1.1-deprecated/go.mod h1:eihoPOH+FgIqa3FpoTwguz/bVUSGBlGQU67vpBeOrBY=
 golang.org/x/tools/go/packages/packagestest v0.1.1-deprecated h1:1h2MnaIAIXISqTFKdENegdpAgUXz6NrPEsbIeWaBRvM=

internal/analysis/analyzer.go

@@ -42,9 +42,10 @@ func Analyze(file *ast.File, info *types.Info, opts Options) []*loopir.Plan {
 			continue
 		}
 		results := loopir.NamedResults(fn, info)
+		reserved := loopir.UsesReservedNames(fn)
 		ast.Inspect(fn.Body, func(n ast.Node) bool {
 			if stmt, ok := n.(ast.Stmt); ok {
-				if plan, _ := analyzeLoop(stmt, info, results, opts.Plan()); plan != nil {
+				if plan, _ := analyzeLoop(stmt, info, info.Scopes[file], results, reserved, opts.Plan()); plan != nil {
 					plans = append(plans, plan)
 				}
 			}
@@ -55,8 +56,10 @@ func Analyze(file *ast.File, info *types.Info, opts Options) []*loopir.Plan {
 }
 
 // analyzeLoop lowers stmt, which must be a for or range statement, and plans
-// it. It returns the reason when the loop is not vectorizable.
-func analyzeLoop(stmt ast.Stmt, info *types.Info, results map[types.Object]bool, plan loopir.Options) (*loopir.Plan, loopir.Reason) {
+// it. It returns the reason when the loop is not vectorizable. reserved is set
+// when the enclosing function uses a name that begins with an underscore (see
+// loopir.UsesReservedNames); file is the scope of the file stmt is in.
+func analyzeLoop(stmt ast.Stmt, info *types.Info, file *types.Scope, results map[types.Object]bool, reserved bool, plan loopir.Options) (*loopir.Plan, loopir.Reason) {
 	switch stmt.(type) {
 	case *ast.RangeStmt, *ast.ForStmt:
 	default:
@@ -66,5 +69,21 @@ func analyzeLoop(stmt ast.Stmt, info *types.Info, results map[types.Object]bool,
 	if reason != "" {
 		return nil, reason
 	}
-	return loopir.NewPlan(loop, plan)
+	p, reason := loopir.NewPlan(loop, plan)
+	if p == nil {
+		return nil, reason
+	}
+	// The names are checked last, so that a loop that cannot be vectorized
+	// anyway keeps the reason that matters.
+	if reserved {
+		return nil, loopir.ReasonReservedName
+	}
+	var scope *types.Scope
+	if file != nil {
+		scope = file.Innermost(stmt.Pos())
+	}
+	if loopir.ShadowsEmittedName(scope, stmt.Pos()) || loopir.HeaderShadowsName(stmt) {
+		return nil, loopir.ReasonShadowedName
+	}
+	return p, reason
 }

internal/analysis/explain.go

@@ -32,6 +32,7 @@ func AnalyzeExplain(fset *token.FileSet, file *ast.File, info *types.Info, opts
 		}
 		isMethod := fn.Recv != nil
 		results := loopir.NamedResults(fn, info)
+		reserved := loopir.UsesReservedNames(fn)
 		ast.Inspect(fn.Body, func(n ast.Node) bool {
 			var body *ast.BlockStmt
 			switch stmt := n.(type) {
@@ -54,7 +55,7 @@ func AnalyzeExplain(fset *token.FileSet, file *ast.File, info *types.Info, opts
 				return true
 			}
 
-			plan, reason := analyzeLoop(n.(ast.Stmt), info, results, opts.Plan())
+			plan, reason := analyzeLoop(n.(ast.Stmt), info, info.Scopes[file], results, reserved, opts.Plan())
 			c.Vectorized = plan != nil
 			c.Reason = reason
 			out = append(out, c)

internal/loopir/lower.go

@@ -7,6 +7,7 @@ import (
 	"go/token"
 	"go/types"
 	"math"
+	"slices"
 	"strconv"
 	"strings"
 )
@@ -286,6 +287,12 @@ func (l *lowerer) bound(e ast.Expr) (bound, bool) {
 	}
 	b := bound{text: types.ExprString(e)}
 	b.value, b.isConst = l.constant(e)
+	if b.isConst {
+		// A named constant or a constant expression (s0, 1 - 1) is spelled as its
+		// value, so the checks that read a constant start or limit see it and the
+		// emitted code never indexes at a negative constant they would have caught.
+		b.text = strconv.FormatInt(b.value, 10)
+	}
 	if call, ok := ast.Unparen(e).(*ast.CallExpr); ok {
 		b.slice, _ = l.lenOf(call)
 	}
@@ -370,6 +377,121 @@ func isIntType(t types.Type) bool {
 	return ok && (basic.Kind() == types.Int || basic.Kind() == types.UntypedInt)
 }
 
+// UsesReservedNames reports whether fn mentions an identifier that begins with
+// an underscore, other than the blank identifier: every name loopvec adds does,
+// so code it inserts into fn could capture one.
+func UsesReservedNames(fn *ast.FuncDecl) bool {
+	found := false
+	ast.Inspect(fn, func(n ast.Node) bool {
+		if id, ok := n.(*ast.Ident); ok && len(id.Name) > 1 && id.Name[0] == '_' {
+			found = true
+		}
+		return !found
+	})
+	return found
+}
+
+// emittedNames are the predeclared and package names the rewritten code uses:
+// len, min, max and copy, uint64 for shift counts, the element types of
+// reductions (byte and rune among them, as go/types spells those), and the
+// simd and unsafe packages.
+var emittedNames = []string{
+	"len", "min", "max", "copy", "uint64",
+	"int8", "int16", "int32", "int64", "uint8", "uint16", "uint32", "float32", "float64",
+	"byte", "rune",
+	"simd", "unsafe",
+}
+
+// HeaderShadowsName reports whether a variable stmt, a for or range statement,
+// declares in its header could capture a name of the rewritten code. The
+// scalar loop that finishes a reduction declares the same variables as stmt and
+// evaluates the bound inside their scope, while stmt evaluates its range
+// operand, or the start it counts down from, outside: an index named len would
+// break len(a), and for n := range n or for n := n - 1; n >= 0; n-- would
+// compare the index with itself. So stmt must not declare one of emittedNames,
+// nor a name used by its range operand or its initial values.
+func HeaderShadowsName(stmt ast.Stmt) bool {
+	var vars, outside []ast.Expr
+	switch s := stmt.(type) {
+	case *ast.RangeStmt:
+		if s.Tok == token.DEFINE {
+			vars = []ast.Expr{s.Key, s.Value}
+		}
+		outside = []ast.Expr{s.X}
+	case *ast.ForStmt:
+		if init, ok := s.Init.(*ast.AssignStmt); ok && init.Tok == token.DEFINE {
+			vars, outside = init.Lhs, init.Rhs
+		}
+	}
+	for _, v := range vars {
+		id, ok := v.(*ast.Ident)
+		if !ok || id.Name == "_" {
+			continue
+		}
+		if slices.Contains(emittedNames, id.Name) {
+			return true
+		}
+		for _, e := range outside {
+			if mentions(e, id.Name) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// mentions reports whether e uses an identifier named name.
+func mentions(e ast.Expr, name string) bool {
+	found := false
+	ast.Inspect(e, func(n ast.Node) bool {
+		if id, ok := n.(*ast.Ident); ok && id.Name == name {
+			found = true
+		}
+		return !found
+	})
+	return found
+}
+
+// helperNames are the predeclared names that _loopvecOverlap, which loopvec
+// declares at package level, uses besides emittedNames.
+var helperNames = []string{"any", "bool", "false", "int", "uintptr"}
+
+// ShadowsEmittedName reports whether, at pos in scope, a name the rewritten code
+// uses means something else than the code needs: a predeclared name declared
+// again in an enclosing scope, the package or an import, or simd or unsafe naming
+// something other than those packages. scope is the innermost scope containing
+// pos, or nil when the type information has no scopes, which counts as shadowed.
+// The names only the overlap helper uses matter at package level, where it is
+// declared.
+func ShadowsEmittedName(scope *types.Scope, pos token.Pos) bool {
+	if scope == nil {
+		return true
+	}
+	pkg := scope
+	for pkg.Parent() != nil && pkg.Parent() != types.Universe {
+		pkg = pkg.Parent()
+	}
+	for _, name := range helperNames {
+		if pkg.Lookup(name) != nil {
+			return true
+		}
+	}
+	for _, name := range emittedNames {
+		_, obj := scope.LookupParent(name, pos)
+		switch name {
+		case "simd", "unsafe":
+			if pkg, ok := obj.(*types.PkgName); obj != nil && (!ok || pkg.Imported().Path() != name) {
+				return true
+			}
+		default:
+			if obj != types.Universe.Lookup(name) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
 // NamedResults returns the named result variables of fn and of every function
 // literal inside it.
 func NamedResults(fn *ast.FuncDecl, info *types.Info) map[types.Object]bool {
@@ -532,6 +654,25 @@ func (l *lowerer) body(node ast.Stmt, block *ast.BlockStmt, trip Trip, step int6
 		}
 	}
 
+	// A constant offset that puts an index below zero makes the scalar loop panic
+	// when it runs: on its first iteration when the start is a constant and the
+	// first index is negative, or on every iteration when the limit is a constant
+	// and even the last index is. It is left alone, since the vector loop would
+	// index or slice at a negative constant, which does not compile.
+	start, errStart := strconv.ParseInt(trip.Start, 10, 64)
+	limit, errLimit := strconv.ParseInt(trip.Limit, 10, 64)
+	negative := func(offset string) bool {
+		off, err := strconv.ParseInt(offset, 10, 64)
+		return err == nil && (errStart == nil && start+off < 0 || errLimit == nil && limit+off <= 0)
+	}
+	for _, stmt := range body {
+		for _, leaf := range Leaves(stmtValue(stmt)) {
+			if load, ok := leaf.(*Load); ok && negative(load.Off) {
+				return nil, ReasonNegativeIndex
+			}
+		}
+	}
+
 	loop := &Loop{
 		Node: node,
 		Ind:  Induction{Var: l.iv, Step: step, Trip: trip},