[go] cmd/compile: "simd" types have non-constant sizeof

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David Chase (Gerrit)

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Jun 9, 2026, 5:23:11 PM (13 hours ago) Jun 9

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David Chase has uploaded the change for review

Commit message

cmd/compile: "simd" types have non-constant sizeof

This is for the simd aka "midway" types whose size is
determined at run time.

Change-Id: I7666ccd8090b066c5b4d434f196ac6c8b8b468f5

Change diff

diff --git a/src/cmd/compile/internal/types2/builtins.go b/src/cmd/compile/internal/types2/builtins.go
index 82c6e9c..f130020 100644
--- a/src/cmd/compile/internal/types2/builtins.go
+++ b/src/cmd/compile/internal/types2/builtins.go
@@ -1034,6 +1034,11 @@
 		t.unpack()
 		varSize := check.hasVarSize(t.rhs())
 
+		// Special case for portable simd types that rewrite to unknown sizes.
+		if o := t.Obj(); o != nil && o.Pkg() != nil && o.Pkg().Name() == "simd" && o.Name() == "_simd" {
+			varSize = true
+		}
+
 		t.mu.Lock()
 		defer t.mu.Unlock()
 
diff --git a/src/simd/sizeof_test.go b/src/simd/sizeof_test.go
new file mode 100644
index 0000000..497d715
--- /dev/null
+++ b/src/simd/sizeof_test.go
@@ -0,0 +1,33 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build goexperiment.simd
+
+package simd_test
+
+import (
+	"simd"
+	"simd/testdata"
+	"testing"
+	"unsafe"
+)
+
+var v simd.Float32s
+
+func TestSizeof(t *testing.T) {
+	var f float32
+	sv0 := int(unsafe.Sizeof(v))
+	sv1 := v.Len() * int(unsafe.Sizeof(f))
+	sV := int(unsafe.Sizeof(testdata.V))
+	sF := int(unsafe.Sizeof(testdata.F()))
+	if sv0 != sv1 {
+		t.Errorf("Expected sv0=%d and sv1=%d are equal, but not", sv0, sv1)
+	}
+	if sF != sv1 {
+		t.Errorf("Expected sF=%d and sv1=%d are equal, but not", sF, sv1)
+	}
+	if sV != sv1 {
+		t.Errorf("Expected sV=%d and sv1=%d are equal, but not", sV, sv1)
+	}
+}
diff --git a/src/simd/testdata/v.go b/src/simd/testdata/v.go
new file mode 100644
index 0000000..4f1710fa
--- /dev/null
+++ b/src/simd/testdata/v.go
@@ -0,0 +1,18 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build goexperiment.simd
+
+package testdata
+
+import (
+	"simd"
+)
+
+var V simd.Float32s
+
+func F() simd.Float32s {
+	var x simd.Float32s
+	return x
+}

Change information

Files:

M src/cmd/compile/internal/types2/builtins.go
A src/simd/sizeof_test.go
A src/simd/testdata/v.go

Change size: M

Delta: 3 files changed, 56 insertions(+), 0 deletions(-)

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Robert Griesemer (Gerrit)

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Jun 9, 2026, 6:23:56 PM (12 hours ago) Jun 9

to David Chase, goph...@pubsubhelper.golang.org, Robert Griesemer, Robert Findley, Gopher Robot, golang...@luci-project-accounts.iam.gserviceaccount.com, golang-co...@googlegroups.com

Attention needed from David Chase, Robert Findley and Robert Griesemer

Robert Griesemer added 3 comments

Patchset-level comments

File-level comment, Patchset 1 (Latest):

Robert Griesemer . unresolved

Seems ok but you need to also fix go/types by calling go generate in that directory.

File src/cmd/compile/internal/types2/builtins.go

Line 1030, Patchset 1 (Latest): check.push(t.obj)

Robert Griesemer . unresolved

maybe here:
```
obj := t.obj
check.push(obj)
```
and then below (line 1038):
```
if pkg := obj.pkg; pkg != nil && ...
```

Line 1038, Patchset 1 (Latest): if o := t.Obj(); o != nil && o.Pkg() != nil && o.Pkg().Name() == "simd" && o.Name() == "_simd" {

Robert Griesemer . unresolved

You should be able to leave away o != nil. All *Named types must have an object (which is where their name is).

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