t27b: f32 @floor, @ceil, @round, @trunc and @rem from libm.t27 (compiler_rt floorf, ceilf, roundf, truncf, fmodf) #7819

Part of #7746 (t27b passes specs/numeric/gf16.t27), which is part of #6063 (t27b coverage), item C10 of #6488. This is the second PR of #7746's split, after #7791.

What gf16.t27 needs

gf16_floor, gf16_ceil, gf16_round, gf16_trunc and gf16_fmod call @floor, @ceil, @round, @trunc and @rem on an f32, and each has direct tests. t27b refuses all five today (ExprCall(@floor) and so on).

What the reference does

Measured on the t27c lab (zig 0.16.0, x86_64 Debug, the CPU has SSE4.1):

• @floor, @ceil and @trunc of an f32 are not calls. Zig emits roundss, which is exact and quiets a signalling NaN (0x7F800001 gives 0x7FC00001). On aarch64, frintm, frintp and frintz do the same.
• @round of an f32 calls compiler_rt roundf: half away from zero, and a NaN comes back as it is (0x7F800001 stays 0x7F800001). aarch64's frinta quiets it instead, so @round of a signalling NaN is the one place the two references differ.
• @rem of an f32 calls compiler_rt fmodf (generic_fmod): exact, and the sign follows the dividend (@rem(-4, 2) is -0.0). Its NaN is (x * y) / (x * y), whose bits are the platform's: 0xFFC00000 on x86_64, 0x7FC00000 on aarch64.

Plan

• Port compiler_rt's floorf, ceilf, roundf, truncf and fmodf to t27 in specs/tri/t27b/libm.t27. For floorf, ceilf and truncf, a NaN is quieted, as the instruction does.
• Add the five builtins to specs/tri/t27b/libm_plan.t27:
  • an f32 calls its routine;
  • an f64 is refused, because libm.t27 has no f64 routine for these yet;
  • @rem takes two operands, and the second takes the first one's type.
• Test each routine against Zig's builtin, bit for bit, over sweeps of bit patterns: signed zeros, subnormals, halves, NaN payloads and the infinities. No NaN bits are asserted where the two references differ.

Acceptance criteria

• t27b test --blockers specs/numeric/gf16.t27 names none of @floor, @ceil, @round, @trunc and @rem.
• libm.t27's new tests pass under t27c test-report with 0 vacuous tests, and under t27b.
• The lab corpus receipt compare gives IMPROVED_ONLY, EQUIVALENT or NEUTRAL.

User Scenarios

• Given gf16.t27 calls @floor, @ceil, @round, @trunc, @rem on f32 values, When t27b compiles gf16.t27, Then none of the five builtins are refused and the tests in gf16.t27 pass.
• Given libm.t27 provides floorf, ceilf, roundf, truncf, fmodf ported from compiler_rt, When t27c test-report runs on libm.t27, Then all new tests pass with 0 vacuous tests and bit-for-bit match Zig's builtins over the specified sweeps.
• Given libm_plan.t27 declares the five f32 builtins with correct signatures, When t27b type-checks a call to @rem with two f32 operands, Then the second operand takes the first's type and f64 calls are refused.

Requirements

• FR-001: specs/tri/t27b/libm.t27 MUST contain floorf, ceilf, roundf, truncf, fmodf routines ported from compiler_rt for f32.
• FR-002: floorf, ceilf, truncf in libm.t27 MUST quiet a signalling NaN (0x7F800001 → 0x7FC00001) matching x86_64 roundss / aarch64 frintm|p|z behavior.
• FR-003: roundf in libm.t27 MUST implement half-away-from-zero and preserve a signalling NaN payload (0x7F800001 stays 0x7F800001) matching compiler_rt roundf.
• FR-004: fmodf in libm.t27 MUST implement exact remainder with sign following dividend (@rem(-4, 2) = -0.0) and produce platform NaN bits via (x*y)/(x*y) matching compiler_rt generic_fmod.
• FR-005: specs/tri/t27b/libm_plan.t27 MUST declare @floor, @ceil, @round, @trunc as unary f32 builtins calling their libm.t27 routines and refuse f64.
• FR-006: specs/tri/t27b/libm_plan.t27 MUST declare @rem as binary f32 builtin where the second operand takes the first's type and refuse f64.
• FR-007: libm.t27 MUST include bit-for-bit tests against Zig builtins covering signed zeros, subnormals, halves, NaN payloads, and infinities with no assertions on differing NaN bits between x86_64 and aarch64.

Success Criteria

• t27b test --blockers specs/numeric/gf16.t27 exits 0 and names none of @floor, @ceil, @round, @trunc, @rem as blockers.
• t27c test-report specs/tri/t27b/libm.t27 exits 0 with 0 vacuous tests and all new routine tests passing.
• t27b test specs/tri/t27b/libm.t27 exits 0.
• Lab corpus receipt compare yields IMPROVED_ONLY, EQUIVALENT, or NEUTRAL (no REGRESSION).

Boundary

• specs/numeric/gf16.t27
• specs/tri/t27b/libm.t27
• specs/tri/t27b/libm_plan.t27