[llvm-bugs] [Bug 36884] New: 32 x 32 but "multhi" generates horible code for MIPS

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=36884

            Bug ID: 36884
           Summary: 32 x 32 but "multhi" generates horible code for MIPS
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: npl at chello.at
                CC: llvm-bugs at lists.llvm.org

Following code was compiled with "-O2 -target mips", also see the compiler
explorer: https://godbolt.org/g/t7C267

Usecase was generating efficient C code for division by 100 with suitable small
inputs. The explicit variant is a tiny bit more efficient for most compilers,
but it totally explodes on MIPS + clang

------ Source -----------------

#include <stdint.h>

uint32_t div100x(uint32_t uintVal, uint32_t *r)
{
    const uint32_t divrem[] = {
        uintVal / 100U,
        uintVal % 100U
    };
    *r = divrem[1];
   return divrem[0];
}

uint32_t div100y(uint32_t uintVal, uint32_t *r)
{
    const uint32_t divrem[] = {
        (uint32_t)(((uint64_t)uintVal * 42949673U) >> 32),
        uintVal - (uint32_t)(((uint64_t)uintVal * 42949673U) >> 32) * 100U
    };
    *r = divrem[1];
   return divrem[0];
}

------- Generated code -------------------

div100y(unsigned int, unsigned int*): # @div100y(unsigned int, unsigned int*)
  addiu $sp, $sp, -8
  sw $fp, 4($sp) # 4-byte Folded Spill
  move $fp, $sp
  sll $1, $4, 3
  addu $2, $1, $4
  sll $3, $4, 5
  addu $6, $3, $2
  sltu $3, $6, $3
  sltu $1, $2, $1
  srl $2, $4, 29
  addu $1, $2, $1
  srl $2, $4, 27
  addu $1, $2, $1
  sll $2, $4, 10
  addu $1, $1, $3
  subu $3, $2, $6
  sll $7, $4, 13
  addu $3, $7, $3
  sll $8, $4, 15
  addu $9, $8, $3
  sll $10, $4, 20
  srl $11, $4, 22
  subu $1, $11, $1
  sltu $2, $2, $6
  subu $6, $10, $9
  srl $11, $4, 7
  sll $12, $4, 25
  srl $13, $4, 9
  sll $14, $4, 23
  srl $15, $4, 12
  srl $24, $4, 17
  srl $25, $4, 19
  subu $1, $1, $2
  addu $1, $25, $1
  sltu $2, $3, $7
  addu $1, $1, $2
  addu $1, $24, $1
  sltu $2, $9, $8
  addu $1, $1, $2
  subu $1, $15, $1
  sltu $2, $10, $9
  subu $1, $1, $2
  addu $2, $14, $6
  addu $1, $13, $1
  addu $3, $12, $2
  sltu $2, $2, $14
  addu $1, $1, $2
  sltu $2, $3, $12
  addu $1, $11, $1
  addu $2, $1, $2
  sll $1, $2, 2
  sll $3, $2, 5
  subu $1, $3, $1
  sll $3, $2, 7
  subu $1, $3, $1
  subu $1, $4, $1
  sw $1, 0($5)
  move $sp, $fp
  lw $fp, 4($sp) # 4-byte Folded Reload
  jr $ra
  addiu $sp, $sp, 8

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