Reuse most of RISCV's implementation with several exceptions:
1. Assign signext/zeroext attribute to args passed in stack.
On RISCV, integer scalars passed in registers have signext/zeroext
when promoted, but are anyext if passed on the stack. This is defined
in early RISCV ABI specification. But after this change [1], integers
should also be signext/zeroext if passed on the stack. So I think
RISCV's ABI lowering should be updated [2].
While in LoongArch ABI spec, we can see that integer scalars narrower
than GRLEN bits are zero/sign-extended no matter passed in registers
or on the stack.
2. Zero-width bit fields are ignored.
This matches GCC's behavior but it hasn't been documented in ABI sepc.
See https://gcc.gnu.org/r12-8294.
3. `char` is signed by default.
There is another difference worth mentioning is that `char` is signed
by default on LoongArch while it is unsigned on RISCV.
This patch also adds `_BitInt` type support to LoongArch and handle it
in LoongArchABIInfo::classifyArgumentType.
[1] cec39a064e
[2] https://github.com/llvm/llvm-project/issues/57261
Differential Revision: https://reviews.llvm.org/D132285
IRgen optimization opportunities.
//===---------------------------------------------------------------------===//
The common pattern of
--
short x; // or char, etc
(x == 10)
--
generates an zext/sext of x which can easily be avoided.
//===---------------------------------------------------------------------===//
Bitfields accesses can be shifted to simplify masking and sign
extension. For example, if the bitfield width is 8 and it is
appropriately aligned then is is a lot shorter to just load the char
directly.
//===---------------------------------------------------------------------===//
It may be worth avoiding creation of alloca's for formal arguments
for the common situation where the argument is never written to or has
its address taken. The idea would be to begin generating code by using
the argument directly and if its address is taken or it is stored to
then generate the alloca and patch up the existing code.
In theory, the same optimization could be a win for block local
variables as long as the declaration dominates all statements in the
block.
NOTE: The main case we care about this for is for -O0 -g compile time
performance, and in that scenario we will need to emit the alloca
anyway currently to emit proper debug info. So this is blocked by
being able to emit debug information which refers to an LLVM
temporary, not an alloca.
//===---------------------------------------------------------------------===//
We should try and avoid generating basic blocks which only contain
jumps. At -O0, this penalizes us all the way from IRgen (malloc &
instruction overhead), all the way down through code generation and
assembly time.
On 176.gcc:expr.ll, it looks like over 12% of basic blocks are just
direct branches!
//===---------------------------------------------------------------------===//
7d88a05cc0