GCC has supported a generic constraint "s" for a long time (since at
least 1992), which references a symbol or label with an optional
constant offset. "i" is a superset that also supports a constant
integer.
GCC's RISC-V port also supports a machine-specific constraint "S",
which cannot be used with a preemptible symbol. (We don't bother to
check preemptibility.) In PIC code, an external symbol is preemptible by
default, making "S" less useful if you want to create an artificial
reference for linker garbage collection, or define sections to hold
symbol addresses:
```
void fun();
// error: impossible constraint in ‘asm’ for riscv64-linux-gnu-gcc -fpie/-fpic
void foo() { asm(".reloc ., BFD_RELOC_NONE, %0" :: "S"(fun)); }
// good even if -fpie/-fpic
void foo() { asm(".reloc ., BFD_RELOC_NONE, %0" :: "s"(fun)); }
```
This patch adds support for "s". Modify https://reviews.llvm.org/D105254
("S") to handle multi-depth GEPs (https://reviews.llvm.org/D61560).
We don't have an AMO instruction for Nand, so with the A extension we
use an LR/SC loop. If we have Zacas we can use a CAS loop instead.
According to the Zacas spec, a CAS loop scales to highly parallel
systems better than LR/SC.
If we can't produce a large enough index vector in i8, we may need to legalize
the shuffle (via scalarization - which in turn gets lowered into stack usage).
This change makes two related changes:
* Deferring legalization until we actually need to generate the vrgather
instruction. With the new recursive structure, this only happens when
doing the fallback for one of the arms.
* Check the actual mask values for something outside of the representable
range.
Both are covered by recently added tests.
Similar to #78403, but for scalable `vwadd(u).wv`, given that #76785 is recommited.
### Code
```
define <vscale x 8 x i64> @vwadd_wv_mask_v8i32(<vscale x 8 x i32> %x, <vscale x 8 x i64> %y) {
%mask = icmp slt <vscale x 8 x i32> %x, shufflevector (<vscale x 8 x i32> insertelement (<vscale x 8 x i32> poison, i32 42, i64 0), <vscale x 8 x i32> poison, <vscale x 8 x i32> zeroinitializer)
%a = select <vscale x 8 x i1> %mask, <vscale x 8 x i32> %x, <vscale x 8 x i32> zeroinitializer
%sa = sext <vscale x 8 x i32> %a to <vscale x 8 x i64>
%ret = add <vscale x 8 x i64> %sa, %y
ret <vscale x 8 x i64> %ret
}
```
### Before this patch
[Compiler Explorer](https://godbolt.org/z/xsoa5xPrd)
```
vwadd_wv_mask_v8i32:
li a0, 42
vsetvli a1, zero, e32, m4, ta, ma
vmslt.vx v0, v8, a0
vmv.v.i v12, 0
vmerge.vvm v24, v12, v8, v0
vwadd.wv v8, v16, v24
ret
```
### After this patch
```
vwadd_wv_mask_v8i32:
li a0, 42
vsetvli a1, zero, e32, m4, ta, ma
vmslt.vx v0, v8, a0
vsetvli zero, zero, e32, m4, tu, mu
vwadd.wv v16, v16, v8, v0.t
vmv8r.v v8, v16
ret
```
It seems that even though we set the size to unknown, there is still an
assumption in alias analysis somewhere that we will only access bytes
*after* the pointer. Since a strided/indexed load/store can have
negative indices, this is not accurate.
This was found in our downstream when the scheduler reordered a strided
load with negative stride above a scalar store that aliased with it.
This patch is aiming at resolving the below missed-optimization case.
### Code
```
define <8 x i64> @vwadd_mask_v8i32(<8 x i32> %x, <8 x i64> %y) {
%mask = icmp slt <8 x i32> %x, <i32 42, i32 42, i32 42, i32 42, i32 42, i32 42, i32 42, i32 42>
%a = select <8 x i1> %mask, <8 x i32> %x, <8 x i32> zeroinitializer
%sa = sext <8 x i32> %a to <8 x i64>
%ret = add <8 x i64> %sa, %y
ret <8 x i64> %ret
}
```
### Before this patch
[Compiler Explorer](https://godbolt.org/z/cd1bKTrx6)
```
vwadd_mask_v8i32:
li a0, 42
vsetivli zero, 8, e32, m2, ta, ma
vmslt.vx v0, v8, a0
vmv.v.i v10, 0
vmerge.vvm v16, v10, v8, v0
vwadd.wv v8, v12, v16
ret
```
### After this patch
```
vwadd_mask_v8i32:
li a0, 42
vsetivli zero, 8, e32, m2, ta, ma
vmslt.vx v0, v8, a0
vsetvli zero, zero, e32, m2, tu, mu
vwadd.wv v12, v12, v8, v0.t
vmv4r.v v8, v12
ret
```
This pattern could be found in a reduction with a widening destination
Specifically, we first do a fold like `(vwadd.wv y, (vmerge cond, x, 0))
-> (vwadd.wv y, x, y, cond)`, then do pattern matching on it.
This fixes a miscompile from #79072 where we were taking the wrong SrcVec to do
the M1 shuffle. E.g. if the SrcVecIdx was 2 and we had 2 VRegsPerSrc, we ended
up taking it from V1 instead of V2.
If we have a bitrotate shuffle, this is also by definition a vreg
splitable shuffle when exact VLEN is known. However, there's no profit
to be had from splitting the wider bitrotate lowering into individual m1
pieces. We'd rather leave it the higher lmul to reduce code size.
This is a general problem for any linear-in-LMUL shuffle expansions when
the vreg splitting still has to do linear work per piece. On first
reflection it seems like element rotation might have the same
interaction, but in that case, splitting can be done via a set of whole
register moves (which may get folded into the consumer depending) which
at least as good as a pair of slideup/slidedown. I think that bitrotate
is the only shuffle expansion we have that actually needs handled here.
If we're lowering an e8 m8 shuffle and we have an index value greater than
255, we have no available space to generate an e16 index vector. The
code had originally handled this correctly, but in a recent refactoring
I had moved the single source code above the check, and thus broke the
single source by accident.
I have a change on review to rework this (https://github.com/llvm/llvm-project/pull/79330), but for now, go with the most obvious fix.
The two single source cases aren't effected by the swap or select matching
as those are dual operand specific. Similarly, a two source shuffle can't
be a rotate.
We can extend this idea for some of the shuffle types above, but some of
them are validly either single or dual source. We don't want to loose that
and the code complexity of versioning early and having to repeat some shuffle
kinds doesn't (currently) seem worth it.
Follow up to 396b6bbc, sink code into consuming branch, and fix one
comment I realized used the misleading wording. (Permute is a specific
sub-type of single source shuffle.)
This builds on bdc41106ee48dce59c500c9a3957af947f30c8c3.
This change completes the migration to a recursive shuffle lowering
strategy where when we encounter an unknown two argument shuffle, we
lower each operand as a single source permute, and then use a vselect
(i.e. a vmerge) to combine the results. This relies for code quality on
the post-isel combine which will aggressively fold that vmerge back into
the materialization of the second operand if possible.
Note: The change includes only the most immediately obvious of the
stylistic cleanup. There's a bunch of code movement that this enables
that I'll do as a separate patch as rolling it into this creates an
unreadable diff.
This patch allows VCIX instructions that have side effect to be
reordered
with memory and other side effecting instructions. However we don't want
VCIX instructions to be reordered with each other, so we propose a dummy
register called VCIX_STATE and make these instructions implicitly define
and use
it.
This patch adds basic TLSDESC support in the RISC-V backend.
Specifically, we add new relocation types for TLSDESC, as prescribed in
https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/373, and add a
new pseudo instruction to simplify code generation.
This patch does not try to optimize the local dynamic case, which can be
improved in separate patches.
Linker side changes will also be handled separately.
The current implementation is only enabled when passing the new
`-enable-tlsdesc` codegen flag.
This is the first step towards an alternate shuffle lowering design for
the general two vector argument case. The goal is to leverage the
existing lowering for single vector permutes to avoid as many of the
vrgathers as required - even if we do need the other.
This patch handles only the first argument, and is arguably a slightly
weird half-step. However, the test changes from the full two argument
recurse patch are a lot harder to reason about. Taking this half step
gives much more easily reviewable changes, and is thus worthwhile. I
intend to post the patch for the second argument once this has landed.
If we have a shuffle which is larger than m1, we may be able to split it
into a series of individual m1 shuffles. This patch starts with the
subcase where the mask allows a 1-to-1 mapping from source register to
destination register - each with a possible permutation of their own. We
can potentially extend this later, thought in practice this seems to
already catch a number of the most interesting cases.
Minor rework of the fallback case for two argument shuffles in lowerVECTOR_SHUFFLE. We had some common code which wasn't actually common, and simplified significantly once specialized for whether we had a select or not.
Although there are predicated versions of minnum/maxnum, the ones for
minimum/maximum are currently missing. This patch introduces these
intrinsics and implements their lowering to RISC-V.
If the element type of the vector we're extracting from doesn't match the type we're
inserting into, we can't directly insert or extract the subvector.
This patch was originally introduced in PR #72340, but was reverted due
to a bug on invalid extension combine.
Specifically, we resolve the case in the
https://github.com/llvm/llvm-project/pull/72340#issuecomment-1874810998
```
define <vscale x 1 x i32> @foo(<vscale x 1 x i1> %x, <vscale x 1 x i2> %y) {
%a = zext <vscale x 1 x i1> %x to <vscale x 1 x i32>
%b = zext <vscale x 1 x i1> %y to <vscale x 1 x i32>
%c = add <vscale x 1 x i32> %a, %b
ret <vscale x 1 x i32> %c
}
```
The previous patch didn't check if the semantic of `ISD::ZERO_EXTEND`
and `ISD::ZERO_EXTEND` is equivalent to the `vsext.vf2` or `vzext.vf2`
(not ensuring the SEW condition on widening Vector Arithmetic
Instructions).
Thanks for @topperc pointing out this bug.
## The original description
This PR mainly aims at resolving the below missed-optimization case,
while it could also be considered as an extension of the previous patch
https://reviews.llvm.org/D133739?id=
### Missed-Optimization Case
Compiler Explorer: https://godbolt.org/z/GzWzP7Pfh
### Source Code:
```
define <vscale x 2 x i16> @multiple_users(ptr %x, ptr %y, ptr %z) {
%a = load <vscale x 2 x i8>, ptr %x
%b = load <vscale x 2 x i8>, ptr %y
%b2 = load <vscale x 2 x i8>, ptr %z
%c = sext <vscale x 2 x i8> %a to <vscale x 2 x i16>
%d = sext <vscale x 2 x i8> %b to <vscale x 2 x i16>
%d2 = sext <vscale x 2 x i8> %b2 to <vscale x 2 x i16>
%e = mul <vscale x 2 x i16> %c, %d
%f = add <vscale x 2 x i16> %c, %d2
%g = sub <vscale x 2 x i16> %c, %d2
%h = or <vscale x 2 x i16> %e, %f
%i = or <vscale x 2 x i16> %h, %g
ret <vscale x 2 x i16> %i
}
```
### Before This Patch
```
# %bb.0:
vsetvli a3, zero, e16, mf2, ta, ma
vle8.v v8, (a0)
vle8.v v9, (a1)
vle8.v v10, (a2)
svf2 v11, v8
vsext.vf2 v8, v9
vsext.vf2 v9, v10
vmul.vv v8, v11, v8
vadd.vv v10, v11, v9
vsub.vv v9, v11, v9
vor.vv v8, v8, v10
vor.vv v8, v8, v9
ret
```
### After This Patch
```
# %bb.0:
vsetvli a3, zero, e8, mf4, ta, ma
vle8.v v8, (a0)
vle8.v v9, (a1)
vle8.v v10, (a2)
vwmul.vv v11, v8, v9
vwadd.vv v9, v8, v10
vwsub.vv v12, v8, v10
vsetvli zero, zero, e16, mf2, ta, ma
vor.vv v8, v11, v9
vor.vv v8, v8, v12
ret
```
We can see Add/Sub/Mul are combined with the Sign Extension.
### Relation to the Patch D133739
The patch D133739 introduced an optimization for folding `ADD_VL`/
`SUB_VL` / `MUL_V` with `VSEXT_VL` / `VZEXT_VL`. However, the patch did
not consider the case of non-fixed length vector case, thus this PR
could also be considered as an extension for the D133739.
If VLEN is exactly known, we may be able to use the vsetivli encoding
instead of the vsetvli a0, zero, <vtype> encoding. This slightly reduces
register pressure.
This builds on 632f1c5, but reverses course a bit. It turns out to be
quite complicated to canonicalize from VLMAX to immediate early because
the sentinel value is widely used in tablegen patterns without knowledge
of LMUL. Instead, we canonicalize towards the VLMAX representation, and
then pick the immediate form during insertion since we have the LMUL
information there.
Within InsertVSETVLI, this could reasonable fit in a couple places. If
reviewers want me to e.g. move it to emission, let me know. Doing so may
require a bit of extra code to e.g. handle comparisons of the two forms,
but shouldn't be too complicated.
This commit includes the necessary changes to clang and LLVM to support
codegen of `RVE` and the `ilp32e`/`lp64e` ABIs.
The differences between `RVE` and `RVI` are:
* `RVE` reduces the integer register count to 16(x0-x16).
* The ABI should be `ilp32e` for 32 bits and `lp64e` for 64 bits.
`RVE` can be combined with all current standard extensions.
The central changes in ilp32e/lp64e ABI, compared to ilp32/lp64 are:
* Only 6 integer argument registers (rather than 8).
* Only 2 callee-saved registers (rather than 12).
* A Stack Alignment of 32bits (rather than 128bits).
* ilp32e isn't compatible with D ISA extension.
If `ilp32e` or `lp64` is used with an ISA that has any of the registers
x16-x31 and f0-f31, then these registers are considered temporaries.
To be compatible with the implementation of ilp32e in GCC, we don't use
aligned registers to pass variadic arguments and set stack alignment\
to 4-bytes for types with length of 2*XLEN.
FastCC is also supported on RVE, while GHC isn't since there is only one
avaiable register.
Differential Revision: https://reviews.llvm.org/D70401
vmv.s.x and vmv.x.s ignore LMUL, so we can replace the PseudoVMV_S_X_MX
and
PseudoVMV_X_S_MX with just one pseudo each. These pseudos use the VR
register
class (just like the actual instruction), so we now only have TableGen
patterns for vectors of LMUL <= 1.
We now rely on the existing combines that shrink LMUL down to 1 for
vmv_s_x_vl (and vfmv_s_f_vl). We could look into removing these combines
later and just inserting the nodes with the correct type in a later
patch.
The test diff is due to the fact that a PseudoVMV_S_X/PsuedoVMV_X_S no
longer
carries any information about LMUL, so if it's the only vector pseudo
instruction in a block then it now defaults to LMUL=1.
Currently vfmv.s.f intrinsics are directly selected to their pseudos via
a
tablegen pattern in RISCVInstrInfoVPseudos.td, whereas the other move
instructions (vmv.s.x/vmv.v.x/vmv.v.f etc.) first get lowered to their
corresponding VL SDNode, then get selected from a pattern in
RISCVInstrInfoVVLPatterns.td
This patch brings vfmv.s.f inline with the other move instructions.
Split out from #71501, where we did this to preserve the behaviour of
selecting
vmv_s_x for VFMV_S_F_VL for small enough immediates.
* Add IRTranslate tests for ADD, SUB, AND, OR, and XOR with scalable
vector types to show that they work as expected.
* Legalize G_ADD, G_SUB, G_AND, G_OR, and G_XOR of scalable vector
type for the RISC-V vector extension.
Similar to #76550, but for `ISD::AVGCEILU`.
Specifically, this patch aims to use `vaaddu` with rounding mode rnu
(i.e `vxrm[1:0] = 0b00`) for `ISD::AVGCEILU`.
### Source code
```
define <vscale x 8 x i8> @vaaddu_vv_nxv8i8_ceil(<vscale x 8 x i8> %x, <vscale x 8 x i8> %y) {
%xzv = zext <vscale x 8 x i8> %x to <vscale x 8 x i16>
%yzv = zext <vscale x 8 x i8> %y to <vscale x 8 x i16>
%add = add nuw nsw <vscale x 8 x i16> %xzv, %yzv
%one = insertelement <vscale x 8 x i16> poison, i16 1, i32 0
%splat = shufflevector <vscale x 8 x i16> %one, <vscale x 8 x i16> poison, <vscale x 8 x i32> zeroinitializer
%add1 = add nuw nsw <vscale x 8 x i16> %add, %splat
%div = lshr <vscale x 8 x i16> %add1, %splat
%ret = trunc <vscale x 8 x i16> %div to <vscale x 8 x i8>
ret <vscale x 8 x i8> %ret
}
```
### Before this patch
```
vaaddu_vv_nxv8i8_ceil:
vsetvli a0, zero, e8, m1, ta, ma
vwaddu.vv v10, v8, v9
vsetvli zero, zero, e16, m2, ta, ma
vadd.vi v10, v10, 1
vsetvli zero, zero, e8, m1, ta, ma
vnsrl.wi v8, v10, 1
ret
```
### After this patch
```
vaaddu_vv_nxv8i8_ceil:
vsetvli a0, zero, e8, m1, ta, ma
csrwi vxrm, 0
vaaddu.vv v8, v8, v9
ret
```
-Rename to GPRPair.
-Rename registers to be named like X10_X11 instead of X10_PD. Except X0
which is now X0_Pair since it is not paired with X1.
-Use unknown size and offset for the subreg indices. This might
be a functional change, but does not affect any lit tests.
This is the logical equivalent for #76710 for APInt and uses the same
naming scheme.
Converted existing users through:
`git grep -l "cast<ConstantSDNode>\(.*\).*getAPIntValueValue" | xargs
sed -E -i
's/cast<ConstantSDNode>\((.*)\)->getAPIntValue/\1->getAsAPIntVal/'`
This follows on from #76708, allowing
`cast<ConstantSDNode>(N)->getZExtValue()` to be replaced with just
`N->getAsZextVal();`
Introduced via `git grep -l "cast<ConstantSDNode>\(.*\).*getZExtValue" |
xargs sed -E -i
's/cast<ConstantSDNode>\((.*)\)->getZExtValue/\1->getAsZExtVal/'` and
then using `git clang-format` on the result.
This patch aims to use `vaaddu` with rounding mode rdn (i.e `vxrm[1:0] =
0b10`) for `ISD::AVGFLOORU`.
### Source code
```
define <8 x i8> @vaaddu_auto(ptr %x, ptr %y, ptr %z) {
%xv = load <8 x i8>, ptr %x, align 2
%yv = load <8 x i8>, ptr %y, align 2
%xzv = zext <8 x i8> %xv to <8 x i16>
%yzv = zext <8 x i8> %yv to <8 x i16>
%add = add nuw nsw <8 x i16> %xzv, %yzv
%div = lshr <8 x i16> %add, <i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1>
%ret = trunc <8 x i16> %div to <8 x i8>
ret <8 x i8> %ret
}
```
### Before this patch
```
vaaddu_auto:
vsetivli zero, 8, e8, mf2, ta, ma
vle8.v v8, (a0)
vle8.v v9, (a1)
vwaddu.vv v10, v8, v9
vnsrl.wi v8, v10, 1
ret
```
### After this patch
```
vaaddu_auto:
vsetivli zero, 8, e8, mf2, ta, ma
vle8.v v8, (a0)
vle8.v v9, (a1)
csrwi vxrm, 2
vaaddu.vv v8, v8, v9
ret
```
### Note on signed averaging addition
Based on the rvv spec, there is also a variant for signed averaging
addition called `vaadd`.
But AFAIU, no matter in which rounding mode, we cannot achieve the
semantic of signed averaging addition through `vaadd`.
Thus this patch only introduces `vaaddu`.
sifive-p450 supports a very restricted version of the short forward
branch optimization from the sifive-7-series.
For sifive-p450, a branch over a single c.mv can be macrofused as a
conditional move operation. Due to encoding restrictions on c.mv, we
can't conditionally move from X0. That would require c.li instead.
Since #72467, `@plt` in assembly output "call foo@plt" is omitted. We
can trivially merge MO_PLT and MO_CALL without any functional change to
assembly/relocatable file output.
Earlier architectures use different call relocation types whether a PLT
is potentially needed: R_386_PLT32/R_386_PC32, R_68K_PLT32/R_68K_PC32,
R_SPARC_WDISP30/R_SPARC_WPLT320. However, as the PLT property is
per-symbol instead of per-call-site and linkers can optimize out a PLT,
the distinction has been confusing.
Arm made good names R_ARM_CALL/R_AARCH64_CALL. Let's use MO_CALL instead
of MO_PLT.
As follow-ups, we can merge fixup_riscv_call/fixup_riscv_call_plt and
VK_RISCV_CALL/VK_RISCV_CALL_PLT.
As far as I can tell if getIndexedAddressParts received an ISD::SUB, the
constant would be negated. So `IsInc` should be set to true since the
SUB was effectively converted to ADD. This means we should never use
PRE_DEC/POST_DEC.
No tests are affected because DAGCombine aggressively turns SUB with
constant into ADD so no lit test has a SUB reach getIndexedAddressParts.
Instruction cost for CodeSize and Latency/RecipThroughput can be very
different. Considering the diversity of CostKind and vendor-specific
cost, and how they are spread across various TTI functions, it's
becoming quite a challenge to handle. This patch adds an interface
getRISCVInstructionCost to address it.
We can use RISCVISD::VMERGE_VL with an undef passthru operand.
I had to rewrite the FMA patterns to handle both undef and non-undef
cases so we can get the tail policy.
This reverts most of commit 5b155aea0e529b7b5c807e189fef6ea5cd5faec9.
I have left the new test file, but regenerated the checks.
This causes failures in our downstream testing. The input types
to the extends need to be checked so we don't create RISCVISD::VZEXT_VL
with illegal or unsupported input type.
This helper function shortens examples like
`cast<ConstantSDNode>(Node->getOperand(1))->getZExtValue();` to
`Node->getConstantOperandVal(1);`.
Implemented with:
`git grep -l
"cast<ConstantSDNode>\(.*->getOperand\(.*\)\)->getZExtValue\(\)" | xargs
sed -E -i
's/cast<ConstantSDNode>\((.*)->getOperand\((.*)\)\)->getZExtValue\(\)/\1->getConstantOperandVal(\2)/`
and `git grep -l
"cast<ConstantSDNode>\(.*\.getOperand\(.*\)\)->getZExtValue\(\)" | xargs
sed -E -i
's/cast<ConstantSDNode>\((.*)\.getOperand\((.*)\)\)->getZExtValue\(\)/\1.getConstantOperandVal(\2)/'`.
With a couple of simple manual fixes needed. Result then processed by
`git clang-format`.
This PR mainly aims at resolving the below missed-optimization case,
while it could also be considered as an extension of the previous patch
https://reviews.llvm.org/D133739?id=
## Missed-Optimization Case
Compiler Explorer: https://godbolt.org/z/GzWzP7Pfh
### Source Code:
```
define <vscale x 2 x i16> @multiple_users(ptr %x, ptr %y, ptr %z) {
%a = load <vscale x 2 x i8>, ptr %x
%b = load <vscale x 2 x i8>, ptr %y
%b2 = load <vscale x 2 x i8>, ptr %z
%c = sext <vscale x 2 x i8> %a to <vscale x 2 x i16>
%d = sext <vscale x 2 x i8> %b to <vscale x 2 x i16>
%d2 = sext <vscale x 2 x i8> %b2 to <vscale x 2 x i16>
%e = mul <vscale x 2 x i16> %c, %d
%f = add <vscale x 2 x i16> %c, %d2
%g = sub <vscale x 2 x i16> %c, %d2
%h = or <vscale x 2 x i16> %e, %f
%i = or <vscale x 2 x i16> %h, %g
ret <vscale x 2 x i16> %i
}
```
### Before This Patch
```
# %bb.0:
vsetvli a3, zero, e16, mf2, ta, ma
vle8.v v8, (a0)
vle8.v v9, (a1)
vle8.v v10, (a2)
svf2 v11, v8
vsext.vf2 v8, v9
vsext.vf2 v9, v10
vmul.vv v8, v11, v8
vadd.vv v10, v11, v9
vsub.vv v9, v11, v9
vor.vv v8, v8, v10
vor.vv v8, v8, v9
ret
```
### After This Patch
```
# %bb.0:
vsetvli a3, zero, e8, mf4, ta, ma
vle8.v v8, (a0)
vle8.v v9, (a1)
vle8.v v10, (a2)
vwmul.vv v11, v8, v9
vwadd.vv v9, v8, v10
vwsub.vv v12, v8, v10
vsetvli zero, zero, e16, mf2, ta, ma
vor.vv v8, v11, v9
vor.vv v8, v8, v12
ret
```
We can see Add/Sub/Mul are combined with the Sign Extension.
## Relation to the Patch D133739
The patch D133739 introduced an optimization for folding `ADD_VL`/
`SUB_VL` / `MUL_V` with `VSEXT_VL` / `VZEXT_VL`. However, the patch did
not consider the case of non-fixed length vector case, thus this PR
could also be considered as an extension for the D133739.
Furthermore, in the current `SelectionDAG`, we represent scalable vector
add (or any binary operator) as a normal `ADD` operation. It might be
better to use an Opcode like `ADD_VL`, which needs further conversation
and decision.
