feat(amd64): floating-point immediates through a synthesised pool

Assisted-by: GLM 5.3 Flash

testdata/verify/floatimm_amd64.s

@@ -0,0 +1,55 @@
+// Floating-point immediates on the SSE scalar paths: the constant is
+// rewritten into a read from a read-only pool symbol ($f64.<hex> or
+// $f32.<hex>, the IEEE-754 bits in the name), RIP-relative with the
+// displacement left to the relocation.  A positive zero on the moves
+// collapses to XORPS dst, dst; a negative zero keeps its sign bit and
+// takes the pool.  The parenthesised $(-1.0) spelling is the one
+// math/floor_amd64.s uses.  Every result is folded back so no
+// instruction is dead.
+
+#include "textflag.h"
+
+// func floatimm(x float64) float64
+TEXT ·floatimm(SB), NOSPLIT, $0-16
+	MOVQ  x+0(FP), AX
+	MOVQ  AX, X0
+	// The floor kernel's sign fold: the parenthesised negative spelling.
+	MOVSD $ (-1.0), X2
+	ANDPD X2, X0
+	// Positive and fractional constants on the scalar moves.
+	MOVSD $1.5, X3
+	MOVSD $0.5, X4
+	MOVSS $2.5, X5
+	MOVSS $-0.5, X6
+	// A positive zero collapses to XORPS; a negative zero does not.
+	MOVSD $0.0, X7
+	MOVSS $0.0, X8
+	MOVSD $-0.0, X9
+	// The scalar arithmetic reads the pool through r/m (hypot's shape).
+	ADDSD $1.0, X3
+	SUBSD $0.5, X4
+	MULSD $-2.5, X4
+	DIVSD $2.0, X3
+	ADDSS $0.25, X5
+	// Fold everything into one double.
+	ADDSD X5, X3
+	ADDSD X6, X3
+	ADDSD X7, X3
+	ADDSD X8, X3
+	ADDSD X9, X3
+	ADDSD X4, X3
+	ADDSD X0, X3
+	MOVSD X3, ret+8(FP)
+	RET
+
+// func floatimmfloat32() float32
+TEXT ·floatimmfloat32(SB), NOSPLIT, $0-4
+	// The single-width pool constants ride the F3 prefix.
+	MOVSS $1.0, X0
+	MOVSS $-1.0, X1
+	MOVSS $0.0, X2
+	ADDSS $0.5, X0
+	ADDSS X1, X0
+	ADDSS X2, X0
+	MOVSS X0, ret+0(FP)
+	RET