To include -O and -Wall in args.

tasks.json

{
    // See https://go.microsoft.com/fwlink/?LinkId=733558 
    // for the documentation about the tasks.json format
    "version": "2.0.0",
    "tasks": [
        {
            "type": "shell",
            "label": "gcc.exe build active file",
            "command": "C:\\mingw-w64\\x86_64-8.1.0-posix-seh-rt_v6-rev0\\mingw64\\bin\\gcc.exe",
            "args": [
                "-g",
                "-O",
                "-Wall",
                "${file}",
                "-o",
                "${fileDirname}\\${fileBasenameNoExtension}.exe"
            ],
            "options": {
                "cwd": "C:\\mingw-w64\\x86_64-8.1.0-posix-seh-rt_v6-rev0\\mingw64\\bin"
            },
            "problemMatcher": [
                "$gcc"
            ],
            "group": {
                "kind": "build",
                "isDefault": true
            }
        }
    ]
}