SSE3 in DC++

The next DC++ release will require SSE3. Steam’s hardware survey currently lists SSE3 as having 99.96% penetration. All AMD and Intel x86 CPUs since the Athlon 64 X2 in 2005 and Intel Core in January 2006 have supported SSE3. Even earlier, though, all Pentium 4 steppings since Prescott which support the NX bit required by Windows 8 and 10 also support SSE3, which extends the effective Intel support back to 2004. I can’t find an Intel CPU which supports NX (required for Win8/10) but not SSE3. Finally, this effectively affects only 32-bit builds, since 64-bit builds exclusively use SSE for floating-point arithmetic.

This effects two basic transformations, one minor and one major, depending on how well the existing code compiles. The minor improvement derives from functions such as bool SettingsDialog::handleClosing() using one instruction rather than two, from

bool SettingsDialog::handleClosing() {
	dwt::Point pt = getWindowSize();
	SettingsManager::getInstance()->set(SettingsManager::SETTINGS_WIDTH,
    cvttss2si eax,DWORD PTR [esp+0x18] ;; eax is just a temporary
    mov    DWORD PTR [edx+0x87c],eax   ;; which is promptly stored to mem

to

bool SettingsDialog::handleClosing() {
	dwt::Point pt = getWindowSize();
	SettingsManager::getInstance()->set(SettingsManager::SETTINGS_WIDTH,
    fisttp DWORD PTR [edx+0x87c]      ;; no byway through eax (also, less register pressure)

However, sometimes cvttss2si and related SSE/SSE2 instructions don’t fit as well, so g++ had been relying on fistp. These instances previously produced terrible code generation; without SSE3, only using through SSE2, part of void SearchFrame::runSearch() compiles to:

	auto llsize = static_cast(lsize);
    fnstcw WORD PTR [ebp-0x50e]     ;; save FP control word to mem
    movzx  eax,WORD PTR [ebp-0x50e] ;; zero-extend-move it to eax
    mov    ah,0xc                   ;; build new control word
    mov    WORD PTR [ebp-0x510],ax  ;; place control word in mem for fldcw
    fld    QWORD PTR [ebp-0x520]    ;; load lsize from mem (same as below)
    fldcw  WORD PTR [ebp-0x510]     ;; load new control word
    fistp  QWORD PTR [ebp-0x548]    ;; with correct control word, round lsize
    fldcw  WORD PTR [ebp-0x50e]     ;; restore previous control word

All 6 red-highlighted lines just scaffold around the actual fistp doing the floating point-to-int rounding, which can cost 80 cycles or more for this single innocuous-looking line of code. By contrast, using fisttp from SSE3, that same fragment collapses to:

	auto llsize = static_cast(lsize);
    fld    QWORD PTR [ebp-0x520]    ;; same as above; load lsize
    fisttp QWORD PTR [ebp-0x548]    ;; convert it. simple.

This pattern recurs many times through DC++, including void AdcHub::handle(AdcCommand::GET which has a portion halving in size and dramatically increasing in speed from

		// Ideal size for m is n * k / ln(2), but we allow some slack
		// When h >= 32, m can't go above 2^h anyway since it's stored in a size_t.
		if(m > (5 * Util::roundUp((int64_t)(n * k / log(2.)), (int64_t)64)) || (h < 32 && m > static_cast(1U << h))) {
    mov    DWORD PTR [esp+0x1c],edi
    xor    ecx,ecx
    imul   eax,DWORD PTR [esp+0x18]
    movd   xmm0,eax
    movq   QWORD PTR [esp+0x58],xmm0
    fild   QWORD PTR [esp+0x58]
    fdiv   QWORD PTR ds:0xca8
    fnstcw WORD PTR [esp+0x22]     ;; same control word dance as before
    movzx  eax,WORD PTR [esp+0x22]
    mov    ah,0xc                  ;; same control word
    mov    WORD PTR [esp+0x20],ax  ;; but fldcw loads from mem not reg
    fldcw  WORD PTR [esp+0x20]     ;; load C and C++-compatible rounding mode
    fistp  QWORD PTR [esp+0x58]    ;; the actual conversion
    fldcw  WORD PTR [esp+0x22]     ;; restore previous
    mov    eax,DWORD PTR [esp+0x58]
    mov    edx,DWORD PTR [esp+0x5c]

to, using the fisttp SSE3 instruction,

		// Ideal size for m is n * k / ln(2), but we allow some slack
		// When h >= 32, m can't go above 2^h anyway since it's stored in a size_t.
		if(m > (5 * Util::roundUp((int64_t)(n * k / log(2.)), (int64_t)64)) || (h < 32 && m > static_cast(1U << h))) {
    mov    DWORD PTR [esp+0x20],edi
    xor    ecx,ecx
    imul   eax,DWORD PTR [esp+0x1c]
    movd   xmm0,eax
    movq   QWORD PTR [esp+0x58],xmm0
    fild   QWORD PTR [esp+0x58]
    fdiv   QWORD PTR ds:0xca8
    fisttp QWORD PTR [esp+0x58]    ;; replaces all seven red lines
    mov    eax,DWORD PTR [esp+0x58]
    mov    edx,DWORD PTR [esp+0x5c]

This specific control word save/convert float/control word restore pattern recurs 19 other times across the current codebase in the dcpp, dwt, and win32 directories, including DownloadManager::getRunningAverage(); HashBloom::get_m(size_t n, size_t k); QueueItem::getDownloadedBytes(); Transfer::getParams(); UploadManager::getRunningAverage(); Grid::calcSizes(…); HashProgressDlg::updateStats(); TransferView::on(HttpManagerListener::Updated, …); and TransferView::onTransferTick(…).

Know your FPU: Fixing Floating Fast provides microbenchmarks showing just how slow this fistp-based technique can be due to the fnstcw/fldcw 80+-cycle FPU pipeline flush and therefore how much faster code which replaces it can become:

Fixed tests...
Testing ANSI fixed() ... Time = 2974.57 ms
Testing fistp fixed()... Time = 3100.84 ms
Testing Sree fixed() ... Time =  606.80 ms

SSE3 provides not simply some hidden code generation aesthetic quality improvement, but a speed increase across much of DC++.

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