[go] simd/archsimd: report CPU features guaranteed by GOAMD64 as constants

[Achille Roussel (Gerrit)]

Achille Roussel has uploaded the change for review

Commit message

simd/archsimd: report CPU features guaranteed by GOAMD64 as constants

The GOAMD64 microarchitecture level guarantees the availability of some
of the CPU features reported by the archsimd package: v3 guarantees AVX,
AVX2, and FMA, and v4 additionally guarantees the AVX-512 features that
make up the combined AVX512 feature (F, BW, CD, DQ, and VL).

Rewrite the corresponding feature checks as

	return goamd64v3 || cpu.X86.HasAVX2

where goamd64v3 and goamd64v4 are constants defined in new files gated
on the amd64.v3 and amd64.v4 build tags that cmd/go derives from
GOAMD64. When the target level guarantees the feature, the check
inlines to a constant true and the compiler removes the code paths
handling the absence of the feature; otherwise the constant folds away
and the check compiles to the same load of the internal/cpu flag as
before.

Update simdgen accordingly so that regenerating cpu.go preserves the
constant checks, and update the generated cpu.go in sync.

Change-Id: Id2522828b51b8d8625c725721e3f5846d93b62a3

Change diff

diff --git a/src/simd/archsimd/_gen/simdgen/gen_simdTypes.go b/src/simd/archsimd/_gen/simdgen/gen_simdTypes.go
index 25d53ed..da7a3af 100644
--- a/src/simd/archsimd/_gen/simdgen/gen_simdTypes.go
+++ b/src/simd/archsimd/_gen/simdgen/gen_simdTypes.go
@@ -217,9 +217,17 @@
 //
 // {{.Feature}} is defined on all GOARCHes, but will only return true on
 // GOARCH {{.GoArch}}.
+{{- if .GOAMD64}}
+//
+// When the program is compiled with GOAMD64=v{{.GOAMD64}} or higher,
+// {{.Feature}} always returns true, allowing the compiler to remove
+// code paths that handle the absence of the feature.
+{{- end}}
 func ({{.FeatureVar}}Features) {{.Feature}}() bool {
 {{- if .Virtual}}
 	return {{range $i, $dep := .Implies}}{{if $i}} && {{end}}cpu.{{$f.FeatureVar}}.Has{{$dep}}{{end}}
+{{- else if .GOAMD64}}
+	return goamd64v{{.GOAMD64}} || cpu.{{.FeatureVar}}.Has{{.Feature}}
 {{- else}}
 	return cpu.{{.FeatureVar}}.Has{{.Feature}}
 {{- end}}
@@ -640,6 +648,12 @@
 	// Virtual means this feature is not represented directly in internal/cpu,
 	// but is instead the logical AND of the features in Implies.
 	Virtual bool
+
+	// GOAMD64 is the lowest GOAMD64 microarchitecture level that guarantees
+	// this feature, or 0 if no level does. When it is non-zero, the feature
+	// check returns true as a compile-time constant if the corresponding
+	// amd64.v<N> build tag is set. Only meaningful on GOARCH amd64.
+	GOAMD64 int
 }
 
 // goarchFeatureInfo maps from GOARCH to CPU feature to additional information
@@ -710,6 +724,7 @@
 		Virtual    bool
 		Implies    []string
 		ImpliesAll string
+		GOAMD64    int
 	}
 	var features []feature
 	for _, k := range featureKeys {
@@ -721,6 +736,7 @@
 			Virtual:    fi.Virtual,
 			Implies:    fi.Implies,
 			ImpliesAll: featureImplies(k.GoArch, k.Feature),
+			GOAMD64:    fi.GOAMD64,
 		})
 	}
 
diff --git a/src/simd/archsimd/_gen/simdgen/xed.go b/src/simd/archsimd/_gen/simdgen/xed.go
index 6e01b66..27250fe 100644
--- a/src/simd/archsimd/_gen/simdgen/xed.go
+++ b/src/simd/archsimd/_gen/simdgen/xed.go
@@ -858,12 +858,17 @@
 	// TODO: In general, Intel doesn't make any guarantees about what flags are
 	// set, so this means our feature checks need to ensure these, just to be
 	// sure.
+	// The GOAMD64 fields record which GOAMD64 microarchitecture level
+	// guarantees each feature: v3 guarantees AVX, AVX2, and FMA, and v4
+	// additionally guarantees the AVX-512 features that make up the
+	// combined "AVX512" feature (F, BW, CD, DQ, and VL).
 	var features = map[string]featureInfo{
-		"AVX2":   {Implies: []string{"AVX"}},
-		"AVX512": {Implies: []string{"AVX2"}},
+		"AVX":    {GOAMD64: 3},
+		"AVX2":   {Implies: []string{"AVX"}, GOAMD64: 3},
+		"AVX512": {Implies: []string{"AVX2"}, GOAMD64: 4},
 
 		"AVXAES": {Virtual: true, Implies: []string{"AVX", "AES"}},
-		"FMA":    {Implies: []string{"AVX"}},
+		"FMA":    {Implies: []string{"AVX"}, GOAMD64: 3},
 		"VAES":   {Implies: []string{"AVX"}},
 
 		// AVX-512 subfeatures.
diff --git a/src/simd/archsimd/cpu.go b/src/simd/archsimd/cpu.go
index 8266bef..69853cd 100644
--- a/src/simd/archsimd/cpu.go
+++ b/src/simd/archsimd/cpu.go
@@ -14,8 +14,12 @@
 //
 // AVX is defined on all GOARCHes, but will only return true on
 // GOARCH amd64.
+//
+// When the program is compiled with GOAMD64=v3 or higher,
+// AVX always returns true, allowing the compiler to remove
+// code paths that handle the absence of the feature.
 func (X86Features) AVX() bool {
-	return cpu.X86.HasAVX
+	return goamd64v3 || cpu.X86.HasAVX
 }
 
 // AVX2 returns whether the CPU supports the AVX2 feature.
@@ -24,8 +28,12 @@
 //
 // AVX2 is defined on all GOARCHes, but will only return true on
 // GOARCH amd64.
+//
+// When the program is compiled with GOAMD64=v3 or higher,
+// AVX2 always returns true, allowing the compiler to remove
+// code paths that handle the absence of the feature.
 func (X86Features) AVX2() bool {
-	return cpu.X86.HasAVX2
+	return goamd64v3 || cpu.X86.HasAVX2
 }
 
 // AVX512 returns whether the CPU supports the AVX512F+CD+BW+DQ+VL features.
@@ -39,8 +47,12 @@
 //
 // AVX512 is defined on all GOARCHes, but will only return true on
 // GOARCH amd64.
+//
+// When the program is compiled with GOAMD64=v4 or higher,
+// AVX512 always returns true, allowing the compiler to remove
+// code paths that handle the absence of the feature.
 func (X86Features) AVX512() bool {
-	return cpu.X86.HasAVX512
+	return goamd64v4 || cpu.X86.HasAVX512
 }
 
 // AVX512BITALG returns whether the CPU supports the AVX512BITALG feature.
@@ -147,8 +159,12 @@
 //
 // FMA is defined on all GOARCHes, but will only return true on
 // GOARCH amd64.
+//
+// When the program is compiled with GOAMD64=v3 or higher,
+// FMA always returns true, allowing the compiler to remove
+// code paths that handle the absence of the feature.
 func (X86Features) FMA() bool {
-	return cpu.X86.HasFMA
+	return goamd64v3 || cpu.X86.HasFMA
 }
 
 // SHA returns whether the CPU supports the SHA feature.
diff --git a/src/simd/archsimd/cpu_amd64v3_off.go b/src/simd/archsimd/cpu_amd64v3_off.go
new file mode 100644
index 0000000..4c3e41a
--- /dev/null
+++ b/src/simd/archsimd/cpu_amd64v3_off.go
@@ -0,0 +1,12 @@
+// 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 && !amd64.v3
+
+package archsimd
+
+// goamd64v3 reports whether the program is compiled for the GOAMD64=v3
+// microarchitecture level or higher, which guarantees the AVX, AVX2,
+// and FMA CPU features.
+const goamd64v3 = false
diff --git a/src/simd/archsimd/cpu_amd64v3_on.go b/src/simd/archsimd/cpu_amd64v3_on.go
new file mode 100644
index 0000000..9cd7ee4
--- /dev/null
+++ b/src/simd/archsimd/cpu_amd64v3_on.go
@@ -0,0 +1,12 @@
+// 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 && amd64.v3
+
+package archsimd
+
+// goamd64v3 reports whether the program is compiled for the GOAMD64=v3
+// microarchitecture level or higher, which guarantees the AVX, AVX2,
+// and FMA CPU features.
+const goamd64v3 = true
diff --git a/src/simd/archsimd/cpu_amd64v4_off.go b/src/simd/archsimd/cpu_amd64v4_off.go
new file mode 100644
index 0000000..023e0ad
--- /dev/null
+++ b/src/simd/archsimd/cpu_amd64v4_off.go
@@ -0,0 +1,12 @@
+// 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 && !amd64.v4
+
+package archsimd
+
+// goamd64v4 reports whether the program is compiled for the GOAMD64=v4
+// microarchitecture level or higher, which guarantees the AVX512F, BW,
+// CD, DQ, and VL CPU features that make up the combined AVX512 feature.
+const goamd64v4 = false
diff --git a/src/simd/archsimd/cpu_amd64v4_on.go b/src/simd/archsimd/cpu_amd64v4_on.go
new file mode 100644
index 0000000..711b9ed
--- /dev/null
+++ b/src/simd/archsimd/cpu_amd64v4_on.go
@@ -0,0 +1,12 @@
+// 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 && amd64.v4
+
+package archsimd
+
+// goamd64v4 reports whether the program is compiled for the GOAMD64=v4
+// microarchitecture level or higher, which guarantees the AVX512F, BW,
+// CD, DQ, and VL CPU features that make up the combined AVX512 feature.
+const goamd64v4 = true

Change information

Files:

• M src/simd/archsimd/_gen/simdgen/gen_simdTypes.go
• M src/simd/archsimd/_gen/simdgen/xed.go
• M src/simd/archsimd/cpu.go
• A src/simd/archsimd/cpu_amd64v3_off.go
• A src/simd/archsimd/cpu_amd64v3_on.go
• A src/simd/archsimd/cpu_amd64v4_off.go
• A src/simd/archsimd/cpu_amd64v4_on.go

Change size: M

Delta: 7 files changed, 92 insertions(+), 7 deletions(-)

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Achille Roussel uploaded new patchset

Achille Roussel uploaded patch set #3 to this change.

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Gerrit-PatchSet: 3

Gerrit-Owner: Achille Roussel <achille...@gmail.com>