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test_asm_avx gates on AVX but executes an AVX2 instruction

Reproduce (compiler rebuilt at HEAD)

$ ./compiler/pascal26 test/test_asm_avx.pas /tmp/avx26   # compiles clean
$ /tmp/avx26
Illegal instruction (core dumped)                        # rc 132

Faulting instruction, from gdb:

Program received signal SIGILL, Illegal instruction.
=> 0x40d2f6:	vbroadcastsd %xmm1,%ymm0

The gap, exactly

The test's header reasoning is right and unusually careful — it gates on three things and explains why each matters:

checked leaf
AVX leaf 1 ecx bit 28
OSXSAVE leaf 1 ecx bit 27
XCR0 SSE + YMM state xgetbv bits 1, 2

and it checks FMA separately and correctly (leaf 1 ecx bit 12 → FmaUsable, used to guard the FMA block at line 177).

What is missing is AVX2 (leaf 7 ebx bit 5). And all 15 vbroadcastsd uses are the register-source form:

vbroadcastsd ymm0, xmm1     { AVX2 — register source }
vbroadcastsd ymm0, [mem]    { AVX1 — memory source }

vbroadcastsd exists in AVX1, which is exactly why this is easy to miss — but only with a memory source. The ymm, xmm form was introduced with AVX2. So on an AVX1-only CPU the gate passes and the first broadcast is #UD, which is the crash the header itself warns about:

"Executing an AVX instruction on a machine without it is #UD — a crash, not a failure message — so a test that assumed AVX would not report 'unsupported', it would take the whole suite down on older hardware."

Correct in principle, one bit short in practice.

The host

plexus is Intel(R) Xeon(R) CPU E5-2620 v2 — Ivy Bridge-EP. Measured:

avx  yes    avx2 NO    fma NO    f16c yes    bmi2 NO

AVX2 and FMA both arrived with Haswell, so this box can never run those paths. Not a hardware problem to fix — the watcher box is what it is, and the test's own design says the answer is to gate.

Fix — either works

  1. Add AVX2 to the gate: leaf 7 (with ecx=0) ebx bit 5, and skip the broadcast paths without it. Consistent with how FMA is already handled.
  2. Use the memory-source form, vbroadcastsd ymm0, [x], which is AVX1 and needs no new gate. Changes what the test exercises, so option 1 is probably truer to intent.

The compiler is not implicated. The VEX emitter produced exactly the right bytes — the header notes the encodings were verified against gas, 25 instructions, 110 bytes, byte-identical. This is only about which CPU may run them.

Gate

test/test_asm_avx.pas exits 0 on plexus (AVX yes, AVX2 no, FMA no) by skipping what it cannot run and saying so, and still exercises the full set on a Haswell or later box. Then the tstate red clears on the next native tier.

Log

Resolution

Both fixes, each where it belongs — the ticket offered them as alternatives, but they answer different halves and taking only one loses something.

The arithmetic blocks broadcast from MEMORY (vbroadcastsd ymm0, a), which is AVX1. So the coverage that was the point of the test — the L bit over four lanes, vcmppd + vmovmskpd proving mask 15, the non-destructive-source check, the 0F38 map — still RUNS on plexus rather than being skipped. Option 1 alone would have made the whole test a no-op on the one box that found the bug.

The register-source form gets its own block behind a real AVX2 gate (Avx2Usable: leaf 7 subleaf 0, ebx bit 5, with a maxLeaf >= 7 guard first — asking for a leaf above the maximum returns the highest leaf's data, a wrong answer rather than zeros). That keeps vbroadcastsd ymm, xmm executed where it is legal, which is what the encoder work was for.

Encodings confirmed against gas, both forms:

gas   c4 e2 7d 19 05 34 12 00 00   vbroadcastsd 0x1234(%rip),%ymm0
gas   c4 e2 7d 19 c1               vbroadcastsd %xmm1,%ymm0
pxx   c4 e2 7d 19 04 25 <abs32>    (memory, SIB-absolute)
pxx   c4 e2 7d 19 c1               (register)

Same VEX prefix, same opcode; only the addressing form differs. The compiler was never implicated and is unchanged.

Verified: passes on this box (AVX2 + FMA present — every block runs), and passes with Avx2Usable/FmaUsable forced False, which is plexus's shape: asm avx ok, rc 0, with the five AVX1 blocks still executing. Gate green (self-host fixedpoint + --tier quick).