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>

3 files changed, 24 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/ssa/_gen/generic.rules b/src/cmd/compile/internal/ssa/_gen/generic.rules
index 58872ca85a..b16aa473cd 100644
--- a/src/cmd/compile/internal/ssa/_gen/generic.rules
+++ b/src/cmd/compile/internal/ssa/_gen/generic.rules
@@ -2079,6 +2079,9 @@
 	&& warnRule(fe.Debug_checknil(), v, "removed nil check")
 	=> ptr
 
+// .dict args are always non-nil.
+(NilCheck ptr:(Arg {sym}) _) && isDictArgSym(sym) => ptr
+
 // Nil checks of nil checks are redundant.
 // See comment at the end of https://go-review.googlesource.com/c/go/+/537775.
 (NilCheck ptr:(NilCheck _ _) _ ) => ptr
diff --git a/src/cmd/compile/internal/ssa/rewrite.go b/src/cmd/compile/internal/ssa/rewrite.go
index 6d83ba5653..880c2223ef 100644
--- a/src/cmd/compile/internal/ssa/rewrite.go
+++ b/src/cmd/compile/internal/ssa/rewrite.go
@@ -6,9 +6,11 @@ package ssa
 
 import (
 	"cmd/compile/internal/base"
+	"cmd/compile/internal/ir"
 	"cmd/compile/internal/logopt"
 	"cmd/compile/internal/reflectdata"
 	"cmd/compile/internal/rttype"
+	"cmd/compile/internal/typecheck"
 	"cmd/compile/internal/types"
 	"cmd/internal/obj"
 	"cmd/internal/obj/s390x"
@@ -2744,3 +2746,7 @@ func panicBoundsCToAux(p PanicBoundsC) Aux {
 func panicBoundsCCToAux(p PanicBoundsCC) Aux {
 	return p
 }
+
+func isDictArgSym(sym Sym) bool {
+	return sym.(*ir.Name).Sym().Name == typecheck.LocalDictName
+}
diff --git a/src/cmd/compile/internal/ssa/rewritegeneric.go b/src/cmd/compile/internal/ssa/rewritegeneric.go
index 7e23194e6a..5e0135be3a 100644
--- a/src/cmd/compile/internal/ssa/rewritegeneric.go
+++ b/src/cmd/compile/internal/ssa/rewritegeneric.go
@@ -21393,6 +21393,21 @@ func rewriteValuegeneric_OpNilCheck(v *Value) bool {
 		v.copyOf(ptr)
 		return true
 	}
+	// match: (NilCheck ptr:(Arg {sym}) _)
+	// cond: isDictArgSym(sym)
+	// result: ptr
+	for {
+		ptr := v_0
+		if ptr.Op != OpArg {
+			break
+		}
+		sym := auxToSym(ptr.Aux)
+		if !(isDictArgSym(sym)) {
+			break
+		}
+		v.copyOf(ptr)
+		return true
+	}
 	// match: (NilCheck ptr:(NilCheck _ _) _ )
 	// result: ptr
 	for {