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Sid Manning dc23f7b29b Add support for Linux/Musl ABI

Differential revision: https://reviews.llvm.org/D72701

The patch adds a new option ABI for Hexagon. It primary deals with
the way variable arguments are passed and is use in the Hexagon Linux Musl
environment.

If a callee function has a variable argument list, it must perform the
following operations to set up its function prologue:

  1. Determine the number of registers which could have been used for passing
     unnamed arguments. This can be calculated by counting the number of
     registers used for passing named arguments. For example, if the callee
     function is as follows:

         int foo(int a, ...){ ... }

     ... then register R0 is used to access the argument ' a '. The registers
     available for passing unnamed arguments are R1, R2, R3, R4, and R5.

  2. Determine the number and size of the named arguments on the stack.

  3. If the callee has named arguments on the stack, it should copy all of these
     arguments to a location below the current position on the stack, and the
     difference should be the size of the register-saved area plus padding
     (if any is necessary).

     The register-saved area constitutes all the registers that could have
     been used to pass unnamed arguments. If the number of registers forming
     the register-saved area is odd, it requires 4 bytes of padding; if the
     number is even, no padding is required. This is done to ensure an 8-byte
     alignment on the stack.  For example, if the callee is as follows:

       int foo(int a, ...){ ... }

     ... then the named arguments should be copied to the following location:

       current_position - 5 (for R1-R5) * 4 (bytes) - 4 (bytes of padding)

     If the callee is as follows:

        int foo(int a, int b, ...){ ... }

     ... then the named arguments should be copied to the following location:

        current_position - 4 (for R2-R5) * 4 (bytes) - 0 (bytes of padding)

  4. After any named arguments have been copied, copy all the registers that
     could have been used to pass unnamed arguments on the stack. If the number
     of registers is odd, leave 4 bytes of padding and then start copying them
     on the stack; if the number is even, no padding is required. This
     constitutes the register-saved area. If padding is required, ensure
     that the start location of padding is 8-byte aligned.  If no padding is
     required, ensure that the start location of the on-stack copy of the
     first register which might have a variable argument is 8-byte aligned.

  5. Decrement the stack pointer by the size of register saved area plus the
     padding.  For example, if the callee is as follows:

        int foo(int a, ...){ ... } ;

     ... then the decrement value should be the following:

        5 (for R1-R5) * 4 (bytes) + 4 (bytes of padding) = 24 bytes

     The decrement should be performed before the allocframe instruction.
     Increment the stack-pointer back by the same amount before returning
     from the function.

2020-01-20 09:59:56 -06:00

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 ; RUN: llc -march=hexagon -mtriple=hexagon-unknown-linux-musl < %s | FileCheck %s
 ; Test that the compiler deallocates the register saved area on Linux
 ; for functions that do not need a frame pointer.
 ; CHECK: r29 = add(r29,#-[[SIZE:[0-9]+]]
 ; CHECK: r29 = add(r29,#[[SIZE]])
 define void @test(...) {
 entry:
   ret void
 }