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compiler: inline slice bounds checking

This improves code size in all tests by about 1% and up to 5% in some
cases, likely because LLVM can better reason about inline bounds checks.
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aykevl authored and deadprogram committed Mar 7, 2019
1 parent 051ad07 commit bd6a7b69cea0f6768252984c82913a861038d381
Showing with 67 additions and 64 deletions.
  1. +61 −36 compiler/asserts.go
  2. +0 −3 compiler/compiler.go
  3. +6 −25 src/runtime/panic.go
@@ -19,33 +19,34 @@ func (c *Compiler) emitLookupBoundsCheck(frame *Frame, arrayLen, index llvm.Valu
return
}

// Sometimes, the index can be e.g. an uint8 or int8, and we have to
// correctly extend that type.
if index.Type().IntTypeWidth() < arrayLen.Type().IntTypeWidth() {
// Sometimes, the index can be e.g. an uint8 or int8, and we have to
// correctly extend that type.
if indexType.(*types.Basic).Info()&types.IsUnsigned == 0 {
index = c.builder.CreateZExt(index, arrayLen.Type(), "")
} else {
index = c.builder.CreateSExt(index, arrayLen.Type(), "")
}
} else if index.Type().IntTypeWidth() > arrayLen.Type().IntTypeWidth() {
// The index is bigger than the array length type, so extend it.
arrayLen = c.builder.CreateZExt(arrayLen, index.Type(), "")
}

// Optimize away trivial cases.
// LLVM would do this anyway with interprocedural optimizations, but it
// helps to see cases where bounds check elimination would really help.
if index.IsConstant() && arrayLen.IsConstant() && !arrayLen.IsUndef() {
index := index.SExtValue()
arrayLen := arrayLen.SExtValue()
if index >= 0 && index < arrayLen {
return
}
}
faultBlock := c.ctx.AddBasicBlock(frame.fn.LLVMFn, "lookup.outofbounds")
nextBlock := c.ctx.AddBasicBlock(frame.fn.LLVMFn, "lookup.next")
frame.blockExits[frame.currentBlock] = nextBlock // adjust outgoing block for phi nodes

if index.Type().IntTypeWidth() > c.intType.IntTypeWidth() {
// Index is too big for the regular bounds check. Use the one for int64.
c.createRuntimeCall("lookupBoundsCheckLong", []llvm.Value{arrayLen, index}, "")
} else {
c.createRuntimeCall("lookupBoundsCheck", []llvm.Value{arrayLen, index}, "")
}
// Now do the bounds check: index >= arrayLen
outOfBounds := c.builder.CreateICmp(llvm.IntUGE, index, arrayLen, "")
c.builder.CreateCondBr(outOfBounds, faultBlock, nextBlock)

// Fail: this is a nil pointer, exit with a panic.
c.builder.SetInsertPointAtEnd(faultBlock)
c.createRuntimeCall("lookuppanic", nil, "")
c.builder.CreateUnreachable()

// Ok: this is a valid pointer.
c.builder.SetInsertPointAtEnd(nextBlock)
}

// emitSliceBoundsCheck emits a bounds check before a slicing operation to make
@@ -57,27 +58,51 @@ func (c *Compiler) emitSliceBoundsCheck(frame *Frame, capacity, low, high llvm.V
return
}

uintptrWidth := c.uintptrType.IntTypeWidth()
if low.Type().IntTypeWidth() > uintptrWidth || high.Type().IntTypeWidth() > uintptrWidth {
if low.Type().IntTypeWidth() < 64 {
if lowType.Info()&types.IsUnsigned != 0 {
low = c.builder.CreateZExt(low, c.ctx.Int64Type(), "")
} else {
low = c.builder.CreateSExt(low, c.ctx.Int64Type(), "")
}
// Extend the capacity integer to be at least as wide as low and high.
capacityType := capacity.Type()
if low.Type().IntTypeWidth() > capacityType.IntTypeWidth() {
capacityType = low.Type()
}
if high.Type().IntTypeWidth() > capacityType.IntTypeWidth() {
capacityType = high.Type()
}
if capacityType != capacity.Type() {
capacity = c.builder.CreateZExt(capacity, capacityType, "")
}

// Extend low and high to be the same size as capacity.
if low.Type().IntTypeWidth() < capacityType.IntTypeWidth() {
if lowType.Info()&types.IsUnsigned != 0 {
low = c.builder.CreateZExt(low, capacityType, "")
} else {
low = c.builder.CreateSExt(low, capacityType, "")
}
if high.Type().IntTypeWidth() < 64 {
if highType.Info()&types.IsUnsigned != 0 {
high = c.builder.CreateZExt(high, c.ctx.Int64Type(), "")
} else {
high = c.builder.CreateSExt(high, c.ctx.Int64Type(), "")
}
}
if high.Type().IntTypeWidth() < capacityType.IntTypeWidth() {
if highType.Info()&types.IsUnsigned != 0 {
high = c.builder.CreateZExt(high, capacityType, "")
} else {
high = c.builder.CreateSExt(high, capacityType, "")
}
// TODO: 32-bit or even 16-bit slice bounds checks for 8-bit platforms
c.createRuntimeCall("sliceBoundsCheck64", []llvm.Value{capacity, low, high}, "")
} else {
c.createRuntimeCall("sliceBoundsCheck", []llvm.Value{capacity, low, high}, "")
}

faultBlock := c.ctx.AddBasicBlock(frame.fn.LLVMFn, "slice.outofbounds")
nextBlock := c.ctx.AddBasicBlock(frame.fn.LLVMFn, "slice.next")
frame.blockExits[frame.currentBlock] = nextBlock // adjust outgoing block for phi nodes

// Now do the bounds check: low > high || high > capacity
outOfBounds1 := c.builder.CreateICmp(llvm.IntUGT, low, high, "slice.lowhigh")
outOfBounds2 := c.builder.CreateICmp(llvm.IntUGT, high, capacity, "slice.highcap")
outOfBounds := c.builder.CreateOr(outOfBounds1, outOfBounds2, "slice.outofbounds")
c.builder.CreateCondBr(outOfBounds, faultBlock, nextBlock)

// Fail: this is a nil pointer, exit with a panic.
c.builder.SetInsertPointAtEnd(faultBlock)
c.createRuntimeCall("slicepanic", nil, "")
c.builder.CreateUnreachable()

// Ok: this is a valid pointer.
c.builder.SetInsertPointAtEnd(nextBlock)
}

// emitNilCheck checks whether the given pointer is nil, and panics if it is. It
@@ -1603,7 +1603,6 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
}

// Bounds check.
// LLVM optimizes this away in most cases.
c.emitLookupBoundsCheck(frame, buflen, index, expr.Index.Type())

switch expr.X.Type().Underlying().(type) {
@@ -1635,7 +1634,6 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
}

// Bounds check.
// LLVM optimizes this away in most cases.
length := c.builder.CreateExtractValue(value, 1, "len")
c.emitLookupBoundsCheck(frame, length, index, expr.Index.Type())

@@ -1883,7 +1881,6 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
low,
}

// This check is optimized away in most cases.
c.emitSliceBoundsCheck(frame, llvmLen, low, high, lowType, highType)

if c.targetData.TypeAllocSize(high.Type()) > c.targetData.TypeAllocSize(c.uintptrType) {
@@ -32,33 +32,14 @@ func nilpanic() {
runtimePanic("nil pointer dereference")
}

// Check for bounds in *ssa.Index, *ssa.IndexAddr and *ssa.Lookup.
func lookupBoundsCheck(length uintptr, index int) {
if index < 0 || index >= int(length) {
runtimePanic("index out of range")
}
}

// Check for bounds in *ssa.Index, *ssa.IndexAddr and *ssa.Lookup.
// Supports 64-bit indexes.
func lookupBoundsCheckLong(length uintptr, index int64) {
if index < 0 || index >= int64(length) {
runtimePanic("index out of range")
}
// Panic when trying to acces an array or slice out of bounds.
func lookuppanic() {
runtimePanic("index out of range")
}

// Check for bounds in *ssa.Slice.
func sliceBoundsCheck(capacity, low, high uintptr) {
if !(0 <= low && low <= high && high <= capacity) {
runtimePanic("slice out of range")
}
}

// Check for bounds in *ssa.Slice. Supports 64-bit indexes.
func sliceBoundsCheck64(capacity uintptr, low, high uint64) {
if !(0 <= low && low <= high && high <= uint64(capacity)) {
runtimePanic("slice out of range")
}
// Panic when trying to slice a slice out of bounds.
func slicepanic() {
runtimePanic("slice out of range")
}

// Check for bounds in *ssa.MakeSlice.

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