[Bug target/104914] New: [MIPS] wrong comparison with scrabbled int value

["mmyangfl at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104914

            Bug ID: 104914
           Summary: [MIPS] wrong comparison with scrabbled int value
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mmyangfl at gmail dot com
  Target Milestone: ---

GCC 12.0 (current git master, 80fcc4b) and 11 generates wrong instructions for
this code. (older version not tested)

$ mips64el-img-elf-gcc -mabi=64 -S -O1 -o - ~/a.c

#include <stdio.h>
void test(const unsigned char *buf) {
  int val;
  ((unsigned char*)&val)[0] = *buf++;
  ((unsigned char*)&val)[1] = *buf++;
  ((unsigned char*)&val)[2] = *buf++;
  ((unsigned char*)&val)[3] = *buf++;
  if(val > 0)
    puts("a");
  else
    fputs("b", stderr);
}

int main() { test("\xff\xff\xff\xff"); }  // => "a"

Generated asm code in question:

test:
        .frame  $sp,16,$31              # vars= 0, regs= 1/0, args= 0, gp= 0
        .mask   0x80000000,-8
        .fmask  0x00000000,0
        .set    noreorder
        .set    nomacro
        daddiu  $sp,$sp,-16
        sd      $31,8($sp)
        lbu     $3,0($4)
        move    $2,$0
        dins    $2,$3,0,8
        lbu     $3,1($4)
        dins    $2,$3,8,8
        lbu     $3,2($4)
        dins    $2,$3,16,8
        lbu     $3,3($4)
        dins    $2,$3,24,8
        blezc   $2,.L2             // signed extending $2 missing!
        lui     $4,%highest(.LC0)
        lui     $2,%hi(.LC0)
        daddiu  $4,$4,%higher(.LC0)
        daddiu  $2,$2,%lo(.LC0)
        dsll    $4,$4,32
        daddu   $4,$4,$2
        balc    puts
        ld      $31,8($sp)
.L5:
        daddiu  $sp,$sp,16
        jrc     $31
.L2:
        ld      $2,%gp_rel(_impure_ptr)($28)
        ld      $5,24($2)
        li      $4,98                   # 0x62
        balc    fputc
        b       .L5
        ld      $31,8($sp)

Below are my attempts to fix this bug:

-fdump-final-insns gives the following statement:

(jump_insn # 0 0 (set (pc)
        (if_then_else (le (reg:SI 2 $2 [orig:201 val ] [201])
                (const_int 0 [0]))
            (label_ref #)
            (pc))) "/home/ding/a.c":8:5# {*branch_ordersi}
     (expr_list:REG_DEAD (reg:SI 2 $2 [orig:201 val ] [201])
        (int_list:REG_BR_PROB 440234148 (nil)))
 -> 2)

After manually `icode != CODE_FOR_cbranchsi4` in gcc/gcc/optabs.cc:4501,
combine pass still combines them back, but the machine description simply
define "cbranch<mode>4" for all cbranch family.

I wonder since MIPS64 can't really do comparsion over partial register, is this
RTL valid?