[Bug target/94173] New: [RISCV] Superfluous stackpointer manipulation

["gcc at gms dot tf" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94173

            Bug ID: 94173
           Summary: [RISCV] Superfluous stackpointer manipulation
           Product: gcc
           Version: 9.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at gms dot tf
  Target Milestone: ---

Created attachment 48033
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48033&action=edit
demonstrate how bar function yields superfluous stackpointer instructions

GCC emits stackpointer decrement/increment instructions in some functions where
they are superfluous.

Example:

struct Pair {
    char *s;
    char *t;
};
typedef struct Pair Pair;

Pair bar(char *s, char *t)
{
    return (Pair){s, t};
}

Expected assembly:

0000000000000002 <bar>:
   2:   8082                    ret


Actual assembly:

0000000000000002 <bar>:
   2:   1141                    addi    sp,sp,-16
   4:   0141                    addi    sp,sp,16
   6:   8082                    ret


Notes:

In an example where the function just returns one value the assembly code
actually just contain the expected assenbly code:


char *foo(char *s, char *t)
{
    return s;
}

Note that per the RISC-V calling-conventions up to 2 integer like values are
returned via the a0/a1 registers, i.e. the same registers where also the first
2 function arguments are placed.


FWIW, Clang 9.0.1 generates for both examples the expected code without
superfluous stackpointer adjustments.


Example code is attched.

I tested with:

riscv64-linux-gnu-gcc -O3 -c -o pair.o pair.c