From 7d3b64a2e372b0e7c550bf9152fe215605e448e4 Mon Sep 17 00:00:00 2001
From: Keith Randall <khr@golang.org>
Date: Sat, 26 Mar 2022 10:03:06 -0700
Subject: [PATCH] cmd/compile: optimize BSRQ with nonzero input

Same as BSFQ, if the input is known to be nonzero we don't need a CMOV
to correct for the weird all-zero behavior of the BSRQ instruction.

Fixes #51964

Change-Id: Ic1d14fdf3e7e698ff2f47efecfd761d8a036012a
Reviewed-on: https://go-review.googlesource.com/c/go/+/396034
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
---
 src/cmd/compile/internal/ssa/gen/AMD64.rules |  2 +-
 src/cmd/compile/internal/ssa/rewriteAMD64.go | 23 ++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/src/cmd/compile/internal/ssa/gen/AMD64.rules b/src/cmd/compile/internal/ssa/gen/AMD64.rules
index 47a6af003c9..87fe0fbee13 100644
--- a/src/cmd/compile/internal/ssa/gen/AMD64.rules
+++ b/src/cmd/compile/internal/ssa/gen/AMD64.rules
@@ -2073,7 +2073,7 @@
 	(CMPXCHGLlock [off1+off2] {sym} ptr old new_ mem)
 
 // We don't need the conditional move if we know the arg of BSF is not zero.
-(CMOVQEQ x _ (Select1 (BSFQ (ORQconst [c] _)))) && c != 0 => x
+(CMOVQEQ x _ (Select1 (BS(F|R)Q (ORQconst [c] _)))) && c != 0 => x
 // Extension is unnecessary for trailing zeros.
 (BSFQ (ORQconst <t> [1<<8] (MOVBQZX x))) => (BSFQ (ORQconst <t> [1<<8] x))
 (BSFQ (ORQconst <t> [1<<16] (MOVWQZX x))) => (BSFQ (ORQconst <t> [1<<16] x))
diff --git a/src/cmd/compile/internal/ssa/rewriteAMD64.go b/src/cmd/compile/internal/ssa/rewriteAMD64.go
index 0c789d6b496..c17d8b03e20 100644
--- a/src/cmd/compile/internal/ssa/rewriteAMD64.go
+++ b/src/cmd/compile/internal/ssa/rewriteAMD64.go
@@ -5191,6 +5191,29 @@ func rewriteValueAMD64_OpAMD64CMOVQEQ(v *Value) bool {
 		v.copyOf(x)
 		return true
 	}
+	// match: (CMOVQEQ x _ (Select1 (BSRQ (ORQconst [c] _))))
+	// cond: c != 0
+	// result: x
+	for {
+		x := v_0
+		if v_2.Op != OpSelect1 {
+			break
+		}
+		v_2_0 := v_2.Args[0]
+		if v_2_0.Op != OpAMD64BSRQ {
+			break
+		}
+		v_2_0_0 := v_2_0.Args[0]
+		if v_2_0_0.Op != OpAMD64ORQconst {
+			break
+		}
+		c := auxIntToInt32(v_2_0_0.AuxInt)
+		if !(c != 0) {
+			break
+		}
+		v.copyOf(x)
+		return true
+	}
 	return false
 }
 func rewriteValueAMD64_OpAMD64CMOVQGE(v *Value) bool {