cmd/compile: add debug-hash flag for fused-multiply-add

This adds a -d debug flag "fmahash" for hashcode search for
floating point architecture-dependent problems. This variable has no
effect on architectures w/o fused-multiply-add.

This was rebased onto the GOSSAHASH renovation so that this could have
its own dedicated environment variable, and so that it would be
cheap (a nil check) to check it in the normal case.

Includes a basic test of the trigger plumbing.

Sample use (on arm64, ppc64le, s390x):

% GOCOMPILEDEBUG=fmahash=001110110 \
  go build -o foo cmd/compile/internal/ssa/testdata/fma.go
fmahash triggered main.main:24 101111101101111001110110
GOFMAHASH triggered main.main:20 010111010000101110111011
1.0000000000000002 1.0000000000000004 -2.220446049250313e-16
exit status 1

The intended use is in conjunction with github.com/dr2chase/gossahash,
which will probably acquire a flag "-fma" to streamline its use. This
tool+use was inspired by an ad hoc use of this technique "in anger"
to debug this very problem.  This is also a dry-run for using this
same technique to identify code sensitive to loop variable
lifetime/capture, should we make that change.

Example intended use, with current search tool (using old environment
variable), for a test example:

gossahash -e GOFMAHASH GOMAGIC=GOFMAHASH go run fma.go
Trying go args=[...], env=[GOFMAHASH=1 GOMAGIC=GOFMAHASH]
go failed (81 distinct triggers): exit status 1
Trying go args=[...], env=[GOFMAHASH=11 GOMAGIC=GOFMAHASH]
go failed (39 distinct triggers): exit status 1
Trying go args=[...], env=[GOFMAHASH=011 GOMAGIC=GOFMAHASH]
go failed (18 distinct triggers): exit status 1
Trying go args=[...], env=[GOFMAHASH=0011 GOMAGIC=GOFMAHASH]
Trying go args=[...], env=[GOFMAHASH=1011 GOMAGIC=GOFMAHASH]
...
Trying go args=[...], env=[GOFMAHASH=0110111011 GOMAGIC=GOFMAHASH]
Trying go args=[...], env=[GOFMAHASH=1110111011 GOMAGIC=GOFMAHASH]
go failed (2 distinct triggers): exit status 1
Trigger string is 'GOFMAHASH triggered math.qzero:427 111111101010011110111011', repeated 6 times
Trigger string is 'GOFMAHASH triggered main.main:20 010111010000101110111011', repeated 1 times
Trying go args=[...], env=[GOFMAHASH=01110111011 GOMAGIC=GOFMAHASH]
go failed (1 distinct triggers): exit status 1
Trigger string is 'GOFMAHASH triggered main.main:20 010111010000101110111011', repeated 1 times
Review GSHS_LAST_FAIL.0.log for failing run
FINISHED, suggest this command line for debugging:
GOSSAFUNC='main.main:20 010111010000101110111011' \
GOFMAHASH=01110111011 GOMAGIC=GOFMAHASH go run fma.go

Change-Id: Ifa22dd8f1c37c18fc8a4f7c396345a364bc367d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/394754
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/base/flag.go

@@ -190,6 +190,10 @@ func ParseFlags() {
 		hashDebug = NewHashDebug("gosshash", Debug.Gossahash, nil)
 	}
 
+	if Debug.Fmahash != "" {
+		FmaHash = NewHashDebug("fmahash", Debug.Fmahash, nil)
+	}
+
 	if Flag.MSan && !platform.MSanSupported(buildcfg.GOOS, buildcfg.GOARCH) {
 		log.Fatalf("%s/%s does not support -msan", buildcfg.GOOS, buildcfg.GOARCH)
 	}