cmd/compile: eliminate nil checks on .dict arg

The first arg of a generic function is the dictionary. This dictionary
is never nil, but it gets a nil check becuase the dict arg is treated as
a slice during construction.

cmp.Compare[go.shape.int] was:

00006 (+41) TESTB AX, (AX)
00007 (+52) CMPQ CX, BX
00008 (52) JGT 14
00009 (+55) JGE 12
00010 (+56) MOVL $1, AX
00011 (56) RET
00012 (+58) XORL AX, AX
00013 (58) RET
00014 (+53) MOVQ $-1, AX
00015 (53) RET

Note how the function begins with a TESTB that loads the dict to perform
the nil check.

This CL eliminates that nil check.

For most generic functions, this doesn't matter too much, but not
infrequently are generic functions written which never actually use the
dictionary (like cmp.Compare), so I suspect this might help in hot code
to avoid repeatedly touching the dictionary in memory, and in cases
where the generic function is not inlined (and thus the dict dropped).

compilecmp shows these changes (deduped):

cmp.Compare[go.shape.float64] 73 -> 72  (-1.37%)
cmp.Compare[go.shape.int] 26 -> 24  (-7.69%)
cmp.Compare[go.shape.int32] 25 -> 23  (-8.00%)
cmp.Compare[go.shape.int64] 26 -> 24  (-7.69%)
cmp.Compare[go.shape.string] 142 -> 141  (-0.70%)
cmp.Compare[go.shape.uint16] 26 -> 24  (-7.69%)
cmp.Compare[go.shape.uint] 26 -> 24  (-7.69%)
cmp.Compare[go.shape.uint32] 25 -> 23  (-8.00%)
cmp.Compare[go.shape.uint64] 26 -> 24  (-7.69%)
cmp.Compare[go.shape.uint8] 25 -> 23  (-8.00%)
cmp.Compare[go.shape.uintptr] 26 -> 24  (-7.69%)
cmp.Less[go.shape.float64] 35 -> 34  (-2.86%)
cmp.Less[go.shape.int32] 8 -> 6  (-25.00%)
cmp.Less[go.shape.int64] 9 -> 7  (-22.22%)
cmp.Less[go.shape.int] 9 -> 7  (-22.22%)
cmp.Less[go.shape.string] 112 -> 110  (-1.79%)
cmp.Less[go.shape.uint16] 9 -> 7  (-22.22%)
cmp.Less[go.shape.uint32] 8 -> 6  (-25.00%)
cmp.Less[go.shape.uint64] 9 -> 7  (-22.22%)
internal/synctest.Associate[go.shape.struct 114 -> 113  (-0.88%)
internal/trace.(*dataTable[go.shape.uint64,go.shape.string]).insert 805 -> 791  (-1.74%)
internal/trace.(*dataTable[go.shape.uint64,go.shape.struct 858 -> 852  (-0.70%)
main.(*gState[go.shape.int64]).stop 2111 -> 2085  (-1.23%)
main.(*gState[go.shape.int64]).unblock 941 -> 923  (-1.91%)
runtime.fmax[go.shape.float32] 85 -> 83  (-2.35%)
runtime.fmax[go.shape.float64] 89 -> 87  (-2.25%)
runtime.fmin[go.shape.float32] 85 -> 83  (-2.35%)
runtime.fmin[go.shape.float64] 89 -> 87  (-2.25%)
slices.BinarySearch[go.shape.[]string,go.shape.string] 346 -> 337  (-2.60%)
slices.Concat[go.shape.[]uint8,go.shape.uint8] 462 -> 453  (-1.95%)
slices.ContainsFunc[go.shape.[]*cmd/vendor/github.com/google/pprof/profile.Sample,go.shape.*uint8] 170 -> 169  (-0.59%)
slices.ContainsFunc[go.shape.[]*debug/dwarf.StructField,go.shape.*uint8] 170 -> 169  (-0.59%)
slices.ContainsFunc[go.shape.[]*go/ast.Field,go.shape.*uint8] 170 -> 169  (-0.59%)
slices.ContainsFunc[go.shape.[]string,go.shape.string] 186 -> 181  (-2.69%)
slices.Contains[go.shape.[]*cmd/compile/internal/syntax.BranchStmt,go.shape.*cmd/compile/internal/syntax.BranchStmt] 44 -> 42  (-4.55%)
slices.Contains[go.shape.[]cmd/compile/internal/syntax.Type,go.shape.interface 223 -> 219  (-1.79%)
slices.Contains[go.shape.[]crypto/tls.CurveID,go.shape.uint16] 44 -> 42  (-4.55%)
slices.Contains[go.shape.[]crypto/tls.SignatureScheme,go.shape.uint16] 44 -> 42  (-4.55%)
slices.Contains[go.shape.[]*go/ast.BranchStmt,go.shape.*go/ast.BranchStmt] 44 -> 42  (-4.55%)
slices.Contains[go.shape.[]go/types.Type,go.shape.interface 223 -> 219  (-1.79%)
slices.Contains[go.shape.[]int,go.shape.int] 44 -> 42  (-4.55%)
slices.Contains[go.shape.[]string,go.shape.string] 223 -> 219  (-1.79%)
slices.Contains[go.shape.[]uint16,go.shape.uint16] 44 -> 42  (-4.55%)
slices.Contains[go.shape.[]uint8,go.shape.uint8] 44 -> 42  (-4.55%)
slices.Insert[go.shape.[]string,go.shape.string] 1189 -> 1170  (-1.60%)
slices.medianCmpFunc[go.shape.struct 1118 -> 1113  (-0.45%)
slices.medianCmpFunc[go.shape.struct 1214 -> 1209  (-0.41%)
slices.medianCmpFunc[go.shape.struct 889 -> 887  (-0.22%)
slices.medianCmpFunc[go.shape.struct 901 -> 874  (-3.00%)
slices.order2Ordered[go.shape.float64] 89 -> 87  (-2.25%)
slices.order2Ordered[go.shape.uint16] 75 -> 70  (-6.67%)
slices.partialInsertionSortOrdered[go.shape.string] 1115 -> 1110  (-0.45%)
slices.partialInsertionSortOrdered[go.shape.uint16] 358 -> 352  (-1.68%)
slices.partitionEqualOrdered[go.shape.int] 208 -> 203  (-2.40%)
slices.partitionEqualOrdered[go.shape.int32] 208 -> 198  (-4.81%)
slices.partitionEqualOrdered[go.shape.int64] 208 -> 203  (-2.40%)
slices.partitionEqualOrdered[go.shape.uint32] 208 -> 198  (-4.81%)
slices.partitionEqualOrdered[go.shape.uint64] 208 -> 203  (-2.40%)
slices.partitionOrdered[go.shape.float64] 538 -> 533  (-0.93%)
slices.partitionOrdered[go.shape.int] 437 -> 427  (-2.29%)
slices.partitionOrdered[go.shape.int64] 437 -> 427  (-2.29%)
slices.partitionOrdered[go.shape.uint16] 447 -> 442  (-1.12%)
slices.partitionOrdered[go.shape.uint64] 437 -> 427  (-2.29%)
slices.rotateCmpFunc[go.shape.struct 1045 -> 1029  (-1.53%)
slices.rotateCmpFunc[go.shape.struct 1205 -> 1163  (-3.49%)
slices.rotateCmpFunc[go.shape.struct 1226 -> 1176  (-4.08%)
slices.rotateCmpFunc[go.shape.struct 1322 -> 1272  (-3.78%)
slices.rotateCmpFunc[go.shape.struct 1419 -> 1400  (-1.34%)
slices.rotateCmpFunc[go.shape.*uint8] 549 -> 538  (-2.00%)
slices.rotateLeft[go.shape.string] 603 -> 588  (-2.49%)
slices.rotateLeft[go.shape.uint8] 255 -> 250  (-1.96%)
slices.siftDownOrdered[go.shape.int] 181 -> 171  (-5.52%)
slices.siftDownOrdered[go.shape.int32] 181 -> 171  (-5.52%)
slices.siftDownOrdered[go.shape.int64] 181 -> 171  (-5.52%)
slices.siftDownOrdered[go.shape.string] 614 -> 592  (-3.58%)
slices.siftDownOrdered[go.shape.uint32] 181 -> 171  (-5.52%)
slices.siftDownOrdered[go.shape.uint64] 181 -> 171  (-5.52%)
time.parseRFC3339[go.shape.string] 1774 -> 1758  (-0.90%)
unique.(*canonMap[go.shape.struct 280 -> 276  (-1.43%)
unique.clone[go.shape.struct 311 -> 293  (-5.79%)
weak.Make[go.shape.6880e4598856efac32416085c0172278cf0fb9e5050ce6518bd9b7f7d1662440] 136 -> 134  (-1.47%)
weak.Make[go.shape.struct 136 -> 134  (-1.47%)
weak.Make[go.shape.uint8] 136 -> 134  (-1.47%)

Change-Id: I43dcea5f2aa37372f773e5edc6a2ef1dee0a8db7
Reviewed-on: https://go-review.googlesource.com/c/go/+/706655
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>

1 files changed, 40 insertions, 0 deletions

diff --git a/test/codegen/generics.go b/test/codegen/generics.go
new file mode 100644
index 0000000000..45c4ca8d6a
--- /dev/null
+++ b/test/codegen/generics.go
@@ -0,0 +1,40 @@
+// asmcheck
+
+// Copyright 2025 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.
+
+package codegen
+
+import "cmp"
+
+func isNaN[T cmp.Ordered](x T) bool {
+	return x != x
+}
+
+func compare[T cmp.Ordered](x, y T) int {
+	// amd64:-"TESTB"
+	// arm64:-"MOVB"
+	xNaN := isNaN(x)
+	yNaN := isNaN(y)
+	if xNaN {
+		if yNaN {
+			return 0
+		}
+		return -1
+	}
+	if yNaN {
+		return +1
+	}
+	if x < y {
+		return -1
+	}
+	if x > y {
+		return +1
+	}
+	return 0
+}
+
+func usesCompare(a, b int) int {
+	return compare(a, b)
+}