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[Windows] Enable tailcall VM on Windows with clang-cl #21619

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henderkes wants to merge 5 commits into from
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[Windows] Enable tailcall VM on Windows with clang-cl #21619

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aa36e75
enable tailcall vm with clang >= 19 on windows (aarch64, x86-64)
henderkes Apr 21, 2026
7e287d5
fix copy/paste mistake
henderkes Apr 21, 2026
4b35586
add test to verify clang 19+ -> tailcall vm
henderkes Apr 21, 2026
01d78ec
Merge remote-tracking branch 'upstream/master' into feat/clang-window…
henderkes Apr 21, 2026
00bfbd0
tailcall VM only for x64
henderkes Apr 21, 2026
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2 changes: 1 addition & 1 deletion Zend/zend_vm_gen.php
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Original file line number Diff line number Diff line change
Expand Up @@ -2497,7 +2497,7 @@ function gen_vm_opcodes_header(
$str .= "# define ZEND_VM_KIND\t\tZEND_VM_KIND_HYBRID\n";
}
if ($GLOBALS["vm_kind_name"][ZEND_VM_GEN_KIND] === "ZEND_VM_KIND_HYBRID" || $GLOBALS["vm_kind_name"][ZEND_VM_GEN_KIND] === "ZEND_VM_KIND_CALL") {
$str .= "#elif defined(HAVE_MUSTTAIL) && defined(HAVE_PRESERVE_NONE) && (defined(__x86_64__) || defined(__aarch64__))\n";
$str .= "#elif defined(HAVE_MUSTTAIL) && defined(HAVE_PRESERVE_NONE) && (defined(__x86_64__) || defined(_M_X64) || defined(__aarch64__) || defined(_M_ARM64))\n";
$str .= "# define ZEND_VM_KIND\t\tZEND_VM_KIND_TAILCALL\n";
$str .= "#else\n";
$str .= "# define ZEND_VM_KIND\t\tZEND_VM_KIND_CALL\n";
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2 changes: 1 addition & 1 deletion Zend/zend_vm_opcodes.h
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22 changes: 22 additions & 0 deletions ext/opcache/jit/zend_jit_ir.c
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Expand Up @@ -3314,6 +3314,27 @@ static PRUNTIME_FUNCTION zend_jit_unwind_callback(DWORD64 pc, PVOID context)

static void zend_jit_setup_unwinder(void)
{
#if ZEND_VM_KIND == ZEND_VM_KIND_TAILCALL
/* TAILCALL VM: fixed_save_regset=0, no registers pushed in prologue.
* fixed_stack_frame_size=40, fixed_call_stack_size=48 (16+IR_SHADOW_ARGS).
* Prologue is: sub rsp, 0x58 (88 bytes = 40+48). */
static const unsigned char uw_data[] = {
0x01, // Version=1, Flags=0
0x04, // Size of prolog (sub rsp,imm8 = 4 bytes: 48 83 ec 58)
0x01, // Count of unwind codes
0x00, // Frame Register=none
0x04, 0xa2, // offset 4: UWOP_ALLOC_SMALL info=10, alloc=(10+1)*8=88
0x00, 0x00, // padding
};
/* Exit call variant: base 88 + 304 (shadow+GP+FP+padding) = 392 (0x188) */
static const unsigned char uw_data_exitcall[] = {
0x01, // Version=1, Flags=0
0x07, // Size of prolog (sub rsp,imm32 = 7 bytes: 48 81 ec 88 01 00 00)
0x02, // Count of unwind codes
0x00, // Frame Register=none
0x07, 0x01, 0x31, 0x00, // offset 7: UWOP_ALLOC_LARGE info=0, size/8=49, alloc=392
};
#else
/* Hardcoded SEH unwind data for JIT-ed PHP functions with "fixed stack frame" */
static const unsigned char uw_data[] = {
0x01, // UBYTE: 3 Version , UBYTE: 5 Flags
Expand Down Expand Up @@ -3348,6 +3369,7 @@ static void zend_jit_setup_unwinder(void)
0x02, 0x50, // 1: pushq %rbp
0x01, 0x30, // 0: pushq %rbx
};
#endif

zend_jit_uw_func = (PRUNTIME_FUNCTION)*dasm_ptr;
*dasm_ptr = (char*)*dasm_ptr + ZEND_MM_ALIGNED_SIZE_EX(sizeof(RUNTIME_FUNCTION) * 4 +
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4 changes: 4 additions & 0 deletions win32/build/confutils.js
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Expand Up @@ -3374,6 +3374,10 @@ function toolset_setup_common_cflags()

var vc_ver = probe_binary(PATH_PROG('cl', null));
ADD_FLAG("CFLAGS"," -fms-compatibility -fms-compatibility-version=" + vc_ver + " -fms-extensions");

if (CLANGVERS >= 1900 && (TARGET_ARCH === 'x64' || TARGET_ARCH === 'arm64')) {
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@henderkes henderkes Apr 21, 2026

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I don't have an ARM64 windows pc to test this with, neither does the CI. Should I perhaps limit this to x64 only as a defensive check for now? We only do x64 releases at FrankenPHP anyhow.

I know clang can do tailcalls on arm64, but I'm not sure if the jit changes work for it.

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@arnaud-lb arnaud-lb Apr 21, 2026

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Maybe @shivammathur is able to test on arm64?

Otherwise it would be fine to limit this to x64 until someone can test.

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@henderkes henderkes Apr 21, 2026

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Oh yeah, that would be great. Could you add the CI: All variations tag please so that the Clang compilation actually runs?

Perhaps we should also add a test to make sure clang on windows actually produces a php binary with ZEND_VM_KIND = tailcall vm?

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@iluuu1994 iluuu1994 Apr 21, 2026

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Oh yeah, that would be great. Could you add the CI: All variations tag please so that the Clang compilation actually runs?

Done, you can push again to retrigger CI.

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@shivammathur shivammathur Apr 21, 2026

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@henderkes There is a windows-11-arm runner you can use to test this.

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@henderkes henderkes Apr 21, 2026

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Fails because the sdk tools don't include an arm64 bat file. I'll just revert the commit and add a safety check to only use tailcall VM on x64.

AC_DEFINE('HAVE_PRESERVE_NONE', 1, 'Whether the compiler supports __attribute__((preserve_none))');
}
}

if (!CLANG_TOOLSET) {
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