[RISCV] CodeGen of RVE and ilp32e/lp64e ABIs (#76777)

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

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Target/RISCV/RISCVFeatures.td b/llvm/lib/Target/RISCV/RISCVFeatures.td
index f4af08fc..fa334c6 100644
--- a/llvm/lib/Target/RISCV/RISCVFeatures.td
+++ b/llvm/lib/Target/RISCV/RISCVFeatures.td
@@ -17,6 +17,13 @@ def HasStdExtZicsr : Predicate<"Subtarget->hasStdExtZicsr()">,
                                 AssemblerPredicate<(all_of FeatureStdExtZicsr),
                                 "'Zicsr' (CSRs)">;
 
+def FeatureStdExtI
+    : SubtargetFeature<"i", "HasStdExtI", "true",
+                       "'I' (Base Integer Instruction Set)">;
+def HasStdExtI : Predicate<"Subtarget->hasStdExtI()">,
+                           AssemblerPredicate<(all_of FeatureStdExtI),
+                           "'I' (Base Integer Instruction Set)">;
+
 def FeatureStdExtM
     : SubtargetFeature<"m", "HasStdExtM", "true",
                        "'M' (Integer Multiplication and Division)">;