From 00ea95eb8dd80813e83515affccc620c005c446c Mon Sep 17 00:00:00 2001
From: tannevaled <tannevaled@users.noreply.github.com>
Date: Sat, 26 Sep 2026 12:25:16 +0200
Subject: [PATCH] Run the 386 lane without an emulator, because qemu-i386 loses
 float state

The 386 lane panicked in go-gfx/gfx at a line the arithmetic makes impossible,
and sampling eight identical lanes per condition on one commit said why:

  under qemu-i386                     5 failures out of 8
  under qemu-i386, asyncpreemptoff=1  0 out of 8
  native on the same runner           0 out of 8
  under qemu-arm                      0 out of 8

Each failure landed on a different input although the seed is fixed, and the
wrong value was a sign flip rather than a rounding difference. qemu-i386 loses
the guest's floating-point state across the signal Go delivers for asynchronous
preemption.

A 386 ELF runs natively on an x86-64 runner, so the emulator bought nothing.
Control that it really ran native: qemu-user-static registers binfmt handlers,
the riscv64 lane took 71 s under both conditions, and the 386 lane went from
~85 s to ~40 s.

arm keeps its emulator: there is no native option and it was clean 8 of 8.
---
 .github/workflows/ci.yml | 29 ++++++++++++++++++++++-------
 1 file changed, 22 insertions(+), 7 deletions(-)

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index aaee26d..20fad41 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -69,12 +69,27 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - { goarch: riscv64, qemu: qemu-riscv64-static }
-          - { goarch: loong64, qemu: qemu-loongarch64-static }
-          - { goarch: ppc64le, qemu: qemu-ppc64le-static }
-          - { goarch: s390x,   qemu: qemu-s390x-static }
-          - { goarch: "386",   qemu: qemu-i386-static }
-          - { goarch: arm,     qemu: qemu-arm-static }
+          - { goarch: riscv64, exec: "-exec=qemu-riscv64-static" }
+          - { goarch: loong64, exec: "-exec=qemu-loongarch64-static" }
+          - { goarch: ppc64le, exec: "-exec=qemu-ppc64le-static" }
+          - { goarch: s390x,   exec: "-exec=qemu-s390x-static" }
+          # 386 runs with NO emulator: a 386 ELF executes directly on the
+          # x86-64 runner. That is not a convenience, it is a correctness
+          # requirement. Measured on one commit of go-gfx/gfx, eight identical
+          # lanes each:
+          #
+          #   under qemu-i386                    5 failures out of 8
+          #   under qemu-i386, asyncpreemptoff=1 0 out of 8
+          #   native on the same runner          0 out of 8
+          #   under qemu-arm                     0 out of 8
+          #
+          # Each failure landed on a DIFFERENT input although the seed is
+          # fixed, and the wrong value was a sign flip rather than a rounding
+          # difference. qemu-i386 loses the guest's floating-point state across
+          # the signal Go delivers for asynchronous preemption. Emulating 386
+          # here would buy nothing and cost a lane that goes red at random.
+          - { goarch: "386",   exec: "" }
+          - { goarch: arm,     exec: "-exec=qemu-arm-static" }
     steps:
       - uses: actions/checkout@v7
       - uses: actions/setup-go@v7
@@ -82,4 +97,4 @@ jobs:
           go-version: "1.27.1"
       - run: sudo apt-get update && sudo apt-get install -y qemu-user-static
       - name: Test under qemu (${{ matrix.goarch }})
-        run: GOARCH=${{ matrix.goarch }} go test -exec=${{ matrix.qemu }} ./...
+        run: GOARCH=${{ matrix.goarch }} go test ${{ matrix.exec }} ./...