cmd/compile,cmd/link: hooks for identifying coverage counters

Add a new "coverage counter" classification for variables to be used
for storing code coverage counter values (somewhat in the same way
that we identify fuzzer counters). Tagging such variables allows us to
aggregate them in the linker, and to treat updates specially.

Updates #51430.

Change-Id: Ib49fb05736ffece98bcc2f7a7c37e991b7f67bbb
Reviewed-on: https://go-review.googlesource.com/c/go/+/401235
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 23 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/inline/inl.go b/src/cmd/compile/internal/inline/inl.go
index a7fd704b85..14adbf5d43 100644
--- a/src/cmd/compile/internal/inline/inl.go
+++ b/src/cmd/compile/internal/inline/inl.go
@@ -37,6 +37,7 @@ import (
 	"cmd/compile/internal/typecheck"
 	"cmd/compile/internal/types"
 	"cmd/internal/obj"
+	"cmd/internal/objabi"
 	"cmd/internal/src"
 )
 
@@ -471,6 +472,28 @@ func (v *hairyVisitor) doNode(n ir.Node) bool {
 				}
 			}
 		}
+
+	case ir.OAS:
+		// Special case for coverage counter updates and coverage
+		// function registrations. Although these correspond to real
+		// operations, we treat them as zero cost for the moment. This
+		// is primarily due to the existence of tests that are
+		// sensitive to inlining-- if the insertion of coverage
+		// instrumentation happens to tip a given function over the
+		// threshold and move it from "inlinable" to "not-inlinable",
+		// this can cause changes in allocation behavior, which can
+		// then result in test failures (a good example is the
+		// TestAllocations in crypto/ed25519).
+		n := n.(*ir.AssignStmt)
+		if n.X.Op() == ir.OINDEX {
+			n := n.X.(*ir.IndexExpr)
+			if n.X.Op() == ir.ONAME && n.X.Type().IsArray() {
+				n := n.X.(*ir.Name)
+				if n.Linksym().Type == objabi.SCOVERAGE_COUNTER {
+					return false
+				}
+			}
+		}
 	}
 
 	v.budget--