feat(riscv64,loong64): PCALIGN, branch relaxation and operand shapes

Assisted-by: GLM 5.3 Flash

testdata/verify/l64forms_loong64.s

@@ -0,0 +1,51 @@
+// The subtract-immediate fold, the TEQ/TNE trap pseudos, PRELDX, the FP
+// condition branches and the N(PC) branch spellings, against the toolchain.
+#include "textflag.h"
+
+// func SubFold(x int64) int64
+TEXT ·SubFold(SB), NOSPLIT, $0-16
+	MOVV  x+0(FP), R8
+	SUBV  $0, R8
+	SUBV  $4, R9, R10
+	SUBV  $4096, R11
+	SUBV  $-4, R12
+	SUB   $1, R13
+	SUBVU $4, R14
+	SUBV  $1048576, R15
+	MOVV  R8, ret+8(FP)
+	RET
+
+// func Traps(x int64) int64
+TEXT ·Traps(SB), NOSPLIT, $0-16
+	MOVV x+0(FP), R4
+	TEQ  $4, R4, R5
+	TEQ  $4, R4
+	TNE  $6, R5, R6
+	MOVV R4, ret+8(FP)
+	RET
+
+// func Prefetch(x int64) int64
+TEXT ·Prefetch(SB), NOSPLIT, $0-16
+	MOVV   x+0(FP), R7
+	PRELDX 0(R7), $0x80001021, $0
+	PRELDX -1(R7), $0x1021, $2
+	MOVV   R7, ret+8(FP)
+	RET
+
+// func BranchForms(x int64) int64
+TEXT ·BranchForms(SB), NOSPLIT, $0-16
+	MOVV x+0(FP), R4
+
+l1:
+	BFPT l1
+	BFPT FCC3, l1
+	BFPF l1
+	JMP  -4(PC)
+	JAL  1(PC)
+	JAL  (R4)
+
+loop:
+	ADDV $1, R4
+	BEQ  R4, R5, loop
+	BNE  R4, l1
+	RET

testdata/verify/pcalign_loong64.s

@@ -0,0 +1,33 @@
+// PCALIGN padding on loong64: andi $0, $0, 0 (the architecture's NOP), plus
+// the automatic loop-head alignment to a 16-byte boundary.
+#include "textflag.h"
+
+// func Pad16(x int64) int64
+TEXT ·Pad16(SB), NOSPLIT, $0-16
+	MOVV    x+0(FP), R4
+	PCALIGN $16
+	ADDV    $1, R4
+	MOVV    R4, ret+8(FP)
+	RET
+
+// func Pad32(x int64) int64
+TEXT ·Pad32(SB), NOSPLIT, $0-16
+	MOVV    x+0(FP), R4
+	PCALIGN $32
+	ADDV    $1, R4
+	MOVV    R4, ret+8(FP)
+	RET
+
+// func LoopAlign(x int64) int64
+TEXT ·LoopAlign(SB), NOSPLIT, $0-16
+	MOVV x+0(FP), R4
+	MOVV $10, R5
+
+loop:
+	BEQ  R4, R5, done
+	ADDV $1, R4
+	JMP  loop
+
+done:
+	MOVV R4, ret+8(FP)
+	RET

testdata/verify/pcalign_riscv64.s

@@ -0,0 +1,35 @@
+// PCALIGN padding on riscv64: 4-byte NOPs with a 2-byte compressed NOP when
+// the pad is 2 mod 4, exactly as the toolchain lays the bytes down.
+#include "textflag.h"
+
+// func Pad8(x int64) int64
+TEXT ·Pad8(SB), NOSPLIT, $0-16
+	MOV     x+0(FP), X5
+	PCALIGN $8
+	ADD     $1, X5
+	MOV     X5, ret+8(FP)
+	RET
+
+// func Pad16(x int64) int64
+TEXT ·Pad16(SB), NOSPLIT, $0-16
+	MOV     x+0(FP), X5
+	PCALIGN $16
+	ADD     $1, X5
+	MOV     X5, ret+8(FP)
+	RET
+
+// func Pad32(x int64) int64
+TEXT ·Pad32(SB), NOSPLIT, $0-16
+	MOV     x+0(FP), X5
+	PCALIGN $32
+	ADD     $1, X5
+	MOV     X5, ret+8(FP)
+	RET
+
+// func PadAfterOdd(x int64) int64
+TEXT ·PadAfterOdd(SB), NOSPLIT, $0-16
+	MOV     x+0(FP), X5
+	PCALIGN $8
+	ADD     $1, X5
+	MOV     X5, ret+8(FP)
+	RET