cmd/compile: disable merge conditional branches pass

It's late in the cycle, let's just disable this new pass and
try again for 1.28.

Update #80102

Change-Id: Idcda2814e22f017999813a52b06013df3735bf18
Reviewed-on: https://go-review.googlesource.com/c/go/+/799700
Reviewed-by: Chencheng Jiang <dorbmons@gmail.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Daniel Morsing <daniel.morsing@gmail.com>

Author: randall77

src/cmd/compile/internal/ssa/compile.go

@@ -503,8 +503,9 @@ var passes = [...]pass{
 	{name: "checkLower", fn: checkLower, required: true},
 	{name: "loop invariant", fn: licm},
 	{name: "late phielim and copyelim", fn: copyelim},
-	{name: "tighten", fn: tighten, required: true},                     // move values closer to their uses
-	{name: "merge conditional branches", fn: mergeConditionalBranches}, // generate conditional comparison instructions on ARM64 architecture
+	{name: "tighten", fn: tighten, required: true}, // move values closer to their uses
+	// TODO: fix 80102 and re-enable.
+	//{name: "merge conditional branches", fn: mergeConditionalBranches}, // generate conditional comparison instructions on ARM64 architecture
 	{name: "late deadcode", fn: deadcode},
 	{name: "critical", fn: critical, required: true}, // remove critical edges
 	{name: "phi tighten", fn: phiTighten},            // place rematerializable phi args near uses to reduce value lifetimes

test/codegen/comparisons.go

@@ -462,7 +462,7 @@ func CmpToZero_ex5(e, f int32, u uint32) int {
 
 func UintLtZero(a uint8, b uint16, c uint32, d uint64) int {
 	// amd64: -`(TESTB|TESTW|TESTL|TESTQ|JCC|JCS)`
-	// arm64: -`(CMPW|CMP|CCMP|CCMPW|BHS|BLO)`
+	// arm64: -`(CMPW|CMP|BHS|BLO)`
 	if a < 0 || b < 0 || c < 0 || d < 0 {
 		return 1
 	}
@@ -471,77 +471,45 @@ func UintLtZero(a uint8, b uint16, c uint32, d uint64) int {
 
 func UintGeqZero(a uint8, b uint16, c uint32, d uint64) int {
 	// amd64: -`(TESTB|TESTW|TESTL|TESTQ|JCS|JCC)`
-	// arm64: -`(CMPW|CMP|CCMP|CCMPW|BLO|BHS)`
+	// arm64: -`(CMPW|CMP|BLO|BHS)`
 	if a >= 0 || b >= 0 || c >= 0 || d >= 0 {
 		return 1
 	}
 	return 0
 }
 
 func UintGtZero(a uint8, b uint16, c uint32, d uint64) int {
-	// arm64: `(CMPW|CMP|CCMP|CCMPW|BNE|BEQ)`
+	// arm64: `(CBN?ZW)` `(CBN?Z[^W])` -`(CMPW|CMP|BLS|BHI)`
 	if a > 0 || b > 0 || c > 0 || d > 0 {
 		return 1
 	}
 	return 0
 }
 
 func UintLeqZero(a uint8, b uint16, c uint32, d uint64) int {
-	// arm64: `(CMPW|CMP|CCMP|CCMPW|BNE|BEQ)`
+	// arm64: `(CBN?ZW)` `(CBN?Z[^W])` -`(CMPW|CMP|BHI|BLS)`
 	if a <= 0 || b <= 0 || c <= 0 || d <= 0 {
 		return 1
 	}
 	return 0
 }
 
 func UintLtOne(a uint8, b uint16, c uint32, d uint64) int {
-	// arm64: `(CMPW|CMP|CCMP|CCMPW|BNE|BEQ)`
+	// arm64: `(CBN?ZW)` `(CBN?Z[^W])` -`(CMPW|CMP|BHS|BLO)`
 	if a < 1 || b < 1 || c < 1 || d < 1 {
 		return 1
 	}
 	return 0
 }
 
 func UintGeqOne(a uint8, b uint16, c uint32, d uint64) int {
-	// arm64: `(CMPW|CMP|CCMP|CCMPW|BNE|BEQ)`
+	// arm64: `(CBN?ZW)` `(CBN?Z[^W])` -`(CMPW|CMP|BLO|BHS)`
 	if a >= 1 || b >= 1 || c >= 1 || d >= 1 {
 		return 1
 	}
 	return 0
 }
 
-func ConditionalCompareUint8(a, b uint8) int {
-	// arm64:"CCMPW EQ, R[0-9]+, [$]1, [$]0"
-	if a == 1 && b == 1 {
-		return 1
-	}
-	return 0
-}
-
-func ConditionalCompareInt16(a, b int16) int {
-	// arm64:"CCMPW LE, R[0-9]+, R[0-9]+, [$]4"
-	if a > 3 || a == b {
-		return 1
-	}
-	return 0
-}
-
-func ConditionalCompareUint32(a, b uint32) int {
-	// arm64:"CCMPW LO, R[0-9]+, [$]28, [$]2"
-	if a > b && a < 28 {
-		return 1
-	}
-	return 0
-}
-
-func ConditionalCompareInt64(a, b int64) int {
-	// arm64:"CCMP GT, R[0-9]+, R[0-9]+, [$]0"
-	if a <= 16 || a != b {
-		return 1
-	}
-	return 0
-}
-
 func CmpToZeroU_ex1(a uint8, b uint16, c uint32, d uint64) int {
 	// wasm:"I64Eqz" -"I64LtU"
 	if 0 < a {