xenia-rs/migration/project-root/ppc-manual/vmx/vcmpgtfp.md

vcmpgtfp — Vector Compare Greater-Than Floating Point

Category: VMX (Altivec) · Form: VC · Opcode: 0x100002c6

Assembler Mnemonics

Mnemonic	XML entry	Flags	Description
vcmpgtfp	vcmpgtfp	—	Vector Compare Greater-Than Floating Point
vcmpgtfp.	vcmpgtfp	Rc=1	Vector Compare Greater-Than Floating Point
vcmpgtfp128	vcmpgtfp128	—	Vector128 Compare Greater-Than Floating-Point
vcmpgtfp128.	vcmpgtfp128	Rc=1	Vector128 Compare Greater-Than Floating-Point

Syntax

vcmpgtfp[Rc] [VD], [VA], [VB]
vcmpgtfp128[Rc] [VD], [VA], [VB]

Encoding

vcmpgtfp — form VC

• Opcode word: 0x100002c6
• Primary opcode (bits 0–5): 4
• Extended opcode: 710
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (4)
6–10	VRT	destination vector register
11–15	VRA	source A
16–20	VRB	source B
21	Rc	record-form flag (updates CR6)
22–31	XO	extended opcode (10 bits)

vcmpgtfp128 — form VX128_R

• Opcode word: 0x18000100
• Primary opcode (bits 0–5): 6
• Extended opcode: 256
• Synchronising: no

Bits	Field	Meaning
0–5	OPCD	primary opcode (4)
6–10	VD128l	destination low 5 bits
11–15	VA128l	source A low 5 bits
16–20	VB128l	source B low 5 bits
21	VA128H	source A high bit
22–25	XO	extended opcode (compare)
26	VA128h	source A middle bit
27	Rc	record-form flag (updates CR6)
28–29	VD128h	destination high 2 bits
30–31	VB128h	source B high 2 bits

Operands

Field	Role	Description
VA	vcmpgtfp: read; vcmpgtfp128: read	Source A vector register.
VB	vcmpgtfp: read; vcmpgtfp128: read	Source B vector register.
VD	vcmpgtfp: write; vcmpgtfp128: write	Destination vector register.
CR	vcmpgtfp: write (conditional); vcmpgtfp128: write (conditional)	Condition-register update. When Rc=1, CR field 0 (or CR6 for vector compares, CR1 for FPU) is updated from the result.

Register Effects

vcmpgtfp

• Reads (always): VA, VB
• Reads (conditional): none
• Writes (always): VD
• Writes (conditional): CR

vcmpgtfp128

• Reads (always): VA, VB
• Reads (conditional): none
• Writes (always): VD
• Writes (conditional): CR

Status-Register Effects

• vcmpgtfp: CR6 ← [all-true, 0, all-false, 0] when Rc=1.
• vcmpgtfp128: CR6 ← [all-true, 0, all-false, 0] when Rc=1.

Operation (pseudocode)

; Pseudocode derives directly from the xenia-rs interpreter
; arm (see Implementation References). Operation semantics:
;   - Read source operands from the fields listed under Operands.
;   - Apply the arithmetic / logical / memory action described
;     in the Description field above.
;   - Write results to the destination register(s); update any
;     status bits enumerated under Status-Register Effects.
; Consult the IBM AIX reference link under IBM Reference for
; canonical PPC-style pseudocode where xenia's expression is
; terse.

C Translation Example

/* C translation: the xenia-rs interpreter arm below in           */
/* Implementation References is the authoritative semantic        */
/* snapshot. Translate it line-by-line:                            */
/*   - ctx.gpr[N]  -> r[N]       (or f[]/v[] for FPRs/VRs)        */
/*   - mem.read_u*/write_u* -> mem_read_u*_be / mem_write_u*_be   */
/*   - ctx.update_cr_signed(fld, v) -> update_cr_signed(fld, v)   */
/*   - ctx.xer_ca / xer_ov / xer_so -> xer.CA / xer.OV / xer.SO   */
/* The Register Effects and Status-Register Effects tables above  */
/* enumerate every side effect a faithful translation must emit.  */

Implementation References

vcmpgtfp

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="vcmpgtfp"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_altivec.cc:639
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:96
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:565
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:2195-2205

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::vcmpgtfp | PpcOpcode::vcmpgtfp128 => {
            let (va, vb, vd) = vmx_reg_triple(instr);
            let a = ctx.vr[va].as_f32x4();
            let b = ctx.vr[vb].as_f32x4();
            let mut r = [0u32; 4];
            for i in 0..4 { r[i] = if a[i] > b[i] { 0xFFFF_FFFF } else { 0 }; }
            ctx.vr[vd] = xenia_types::Vec128::from_u32x4_array(r);
            let rc = if matches!(instr.opcode, PpcOpcode::vcmpgtfp128) { instr.vx128r_rc_bit() } else { instr.vc_rc_bit() };
            if rc { update_cr6_from_vmask(&r, ctx); }
            ctx.pc += 4;
        }

vcmpgtfp128

• xenia-canary XML: tools/ppc-instructions.xml — search for mnem="vcmpgtfp128"
• xenia-canary emit: src/xenia/cpu/ppc/ppc_emit_altivec.cc:643
• xenia-rs opcode: crates/xenia-cpu/src/opcode.rs:96
• xenia-rs decoder: crates/xenia-cpu/src/decoder.rs:683
• xenia-rs interpreter: crates/xenia-cpu/src/interpreter.rs:2195-2205

xenia-rs interpreter body (frozen snapshot)

        PpcOpcode::vcmpgtfp | PpcOpcode::vcmpgtfp128 => {
            let (va, vb, vd) = vmx_reg_triple(instr);
            let a = ctx.vr[va].as_f32x4();
            let b = ctx.vr[vb].as_f32x4();
            let mut r = [0u32; 4];
            for i in 0..4 { r[i] = if a[i] > b[i] { 0xFFFF_FFFF } else { 0 }; }
            ctx.vr[vd] = xenia_types::Vec128::from_u32x4_array(r);
            let rc = if matches!(instr.opcode, PpcOpcode::vcmpgtfp128) { instr.vx128r_rc_bit() } else { instr.vc_rc_bit() };
            if rc { update_cr6_from_vmask(&r, ctx); }
            ctx.pc += 4;
        }

Special Cases & Edge Conditions

• Per-lane mask: all-ones / all-zero. Four word lanes; VD[i] = (VA[i] > VB[i]) ? 0xFFFFFFFF : 0.
• NaN handling: false. Any NaN input produces a false lane (no sticky flag, no exception) — matches IEEE-754 quiet-compare.
• +0 > -0 is false. Zero signs ignored.
• VSCR[NJ] denormals. With NJ = 1, denormal inputs flush to zero before the compare.
• CR6 update when Rc=1 (vcmpgtfp.). CR6 = [lt = all-true, gt = 0, eq = all-false, so = 0]. Use bc 12,24 for "all-greater" branches and bc 12,26 for "no-lane-greater".
• Compose with vsel. Mask plus vsel implements per-lane if (a > b) x else y.
• No VSCR[SAT], no XER changes, no traps.
• VMX128 sibling (vcmpgtfp128). Identical semantics with the extended encoding.

Related Instructions

• vcmpgefp — >= for floats.
• vcmpeqfp — equality for floats.
• vcmpbfp — bounds check (|VA| <= |VB|).
• vsel, vand, vandc, vor, vxor — mask consumers / combinators.
• vmaxfp, vminfp — direct max / min.

IBM Reference

• AIX 7.3 — vcmpgtfp (Vector Compare Greater-Than Floating Point)
• IBM AltiVec Technology Programmer's Interface Manual, Chapter 5 — Floating-Point Compares