On Sun, 2023-11-19 at 17:47 -0700, Jeff Law wrote:
> This is work originally started by Joern @ Embecosm.
>
> There's been a long standing sense that we're generating too many
> sign/zero extensions on the RISC-V port. REE is useful, but it's really
> focused on a relatively narrow part of the extension problem.
>
> What Joern's patch does is introduce a new pass which tracks liveness of
> chunks of pseudo regs. Specifically it tracks bits 0..7, 8..15, 16..31
> and 32..63.
>
> If it encounters a sign/zero extend that sets bits that are never read,
> then it replaces the sign/zero extension with a narrowing subreg. The
> narrowing subreg usually gets eliminated by subsequent passes (it's just
> a copy after all).
>
> Jivan has done some analysis and found that it eliminates roughly 1% of
> the dynamic instruction stream for x264 as well as some redundant
> extensions in the coremark benchmark (both on rv64). In my own testing
> as I worked through issues on other architectures I clearly saw it
> helping in various places within GCC itself or in the testsuite.
>
> The basic structure is to first do a fairly standard liveness analysis
> on the chunks, seeding original state with the liveness data from DF.
> Once that's stable, we do a final pass to identify the useless
> extensions and transform them into narrowing subregs.
>
> A few key points to remember.
>
> For destination processing it is always safe to ignore a destination.
> Ignoring a destination merely means that whatever was live after the
> given insn will continue to be live before the insn. What is not safe
> is to clear a bit in the LIVENOW bitmap for a destination chunk that is
> not set. This comes into play with things like STRICT_LOW_PART.
>
> For source processing the safe thing to do is to set all the chunks in a
> register as live. It is never safe to fail to process a source operand.
>
> When a destination object is not fully live, we try to transfer that
> limited liveness to the source operands. So for example if bits 16..63
> are dead in a destination of a PLUS, we need not mark bits 16..63 as
> live for the source operands. We have to be careful -- consider a shift
> count on a target without SHIFT_COUNT_TRUNCATED set. So we have both a
> list of RTL codes where we can transfer liveness and a few codes where
> one of the operands may need to be fully live (ex, a shift count) while
> the other input may not need to be fully live (value left shifted).
>
> Locally we have had this enabled at -O1 and above to encourage testing,
> but I'm thinking that for the trunk enabling at -O2 and above is the
> right thing to do.
>
> This has (of course) been tested on rv64. It's also been bootstrapped
> and regression tested on x86. Bootstrap and regression tested (C only)
> for m68k, sh4, sh4eb, alpha. Earlier versions were also bootstrapped
> and regression tested on ppc, hppa and s390x (C only for those as well).
> It's also been tested on the various crosses in my tester. So we've
> got reasonable coverage of 16, 32 and 64 bit targets, big and little
> endian, with and without SHIFT_COUNT_TRUNCATED and all kinds of other
> oddities.
>
> The included tests are for RISC-V only because not all targets are going
> to have extraneous extensions. There's tests from coremark, x264 and
> GCC's bz database. It probably wouldn't be hard to add aarch64
> testscases. The BZs listed are improved by this patch for aarch64.
>
> Given the amount of work Jivan and I have done, I'm not comfortable
> self-approving at this time. I'd much rather have another set of eyes
> on the code. Hopefully the code is documented well enough for that to
> be useful exercise.
>
> So, no need to work from Pago Pago for this patch. I may make another
> attempt at the eswin conditional move work while working virtually in
> Pago Pago though.
>
> Thoughts, comments, recommendations?
Unfortunately, I get some ICE building stage 1 libgcc with this patch on
loongarch64-linux-gnu:
during RTL pass: ext_dce
../../../gcc/libgcc/libgcc2.c: In function ‘__absvdi2’:
../../../gcc/libgcc/libgcc2.c:224:1: internal compiler error: Segmentation fault
224 | }
| ^
0x120baa477 crash_signal
../../gcc/gcc/toplev.cc:316
0x1216aeeb4 ext_dce_process_sets
../../gcc/gcc/ext-dce.cc:128
0x1216afbaf ext_dce_process_bb
../../gcc/gcc/ext-dce.cc:647
0x1216afbaf ext_dce
../../gcc/gcc/ext-dce.cc:802
0x1216afbaf execute
../../gcc/gcc/ext-dce.cc:868
Please submit a full bug report, with preprocessed source (by using
-freport-bug).
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.
--
Xi Ruoyao <[email protected]>
School of Aerospace Science and Technology, Xidian University
