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Introduce preliminary macro operation fusion #132

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May 29, 2023
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Introduce preliminary macro operation fusion #132

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17 changes: 16 additions & 1 deletion src/decode.h
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Original file line number Diff line number Diff line change
Expand Up @@ -156,7 +156,14 @@
_(cjalr, 1) \
_(cadd, 0) \
_(cswsp, 0) \
)
) \
/* macro operation fusion: convert specific RISC-V instruction patterns
* into faster and equivalent code
*/ \
_(fuse1, 0) \
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_(fuse2, 0) \
_(fuse3, 0) \
_(fuse4, 0)
/* clang-format on */

/* IR list */
Expand Down Expand Up @@ -228,6 +235,11 @@ enum {
INSN_32 = 4,
};

typedef struct {
int32_t imm;
uint8_t rd, rs1, rs2;
} opcode_fuse_t;

typedef struct rv_insn {
union {
int32_t imm;
Expand All @@ -240,6 +252,9 @@ typedef struct rv_insn {
#if RV32_HAS(EXT_C)
uint8_t shamt;
#endif
/* fuse operation */
int16_t imm2;
opcode_fuse_t *fuse;

/* instruction length */
uint8_t insn_len;
Expand Down
137 changes: 135 additions & 2 deletions src/emulate.c
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Expand Up @@ -31,6 +31,7 @@ extern struct target_ops gdbstub_ops;
#include "decode.h"
#include "riscv.h"
#include "riscv_private.h"
#include "state.h"
#include "utils.h"

/* RISC-V exception code list */
Expand Down Expand Up @@ -310,7 +311,15 @@ static uint32_t last_pc = 0;
/* RV32I Base Instruction Set */

/* Internal */
RVOP(nop, {/* no operation */});
static bool do_nop(riscv_t *rv, const rv_insn_t *ir)
{
rv->X[rv_reg_zero] = 0;
rv->csr_cycle++;
rv->PC += ir->insn_len;
const rv_insn_t *next = ir + 1;
MUST_TAIL return next->impl(rv, next);
}


/* LUI is used to build 32-bit constants and uses the U-type format. LUI
* places the U-immediate value in the top 20 bits of the destination
Expand Down Expand Up @@ -1219,6 +1228,46 @@ RVOP(cswsp, {
})
#endif

/* auipc + addi */
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@jserv jserv May 27, 2023 •
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Is it possible to manipulate the sequence lui + addi?
See #81 (comment)

Disassembly of CoreMark:

   10324:       000087b7                lui     a5,0x8
   10328:       b0578793                addi    a5,a5,-1275 # 0x7b05

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It is possible. however, there are some problems when running qrcode.elf if we import this pattern, so I skip it in this pull request.

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It is possible. however, there are some problems when running qrcode.elf if we import this pattern, so I skip it in this pull request.

Add a comment starting with "FIXME: lui + addi"

RVOP(fuse1, { rv->X[ir->rd] = (int32_t) (rv->PC + ir->imm + ir->imm2); })

/* auipc + add */
RVOP(fuse2, {
rv->X[ir->rd] = (int32_t) (rv->X[ir->rs1]) + (int32_t) (rv->PC + ir->imm);
})

/* multiple sw */
RVOP(fuse3, {
opcode_fuse_t *fuse = ir->fuse;
uint32_t addr = rv->X[fuse[0].rs1] + fuse[0].imm;
/* the memory addresses of the sw instructions are contiguous, so we only
* need to check the first sw instruction to determine if its memory address
* is misaligned or if the memory chunk does not exist.
*/
RV_EXC_MISALIGN_HANDLER(3, store, false, 1);
rv->io.mem_write_w(rv, addr, rv->X[fuse[0].rs2]);
for (int i = 1; i < ir->imm2; i++) {
addr = rv->X[fuse[i].rs1] + fuse[i].imm;
rv->io.mem_write_w(rv, addr, rv->X[fuse[i].rs2]);
}
})

/* multiple lw */
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lw is the most frequent instruction (see #34), and we might dive into its use case more.

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Can you handle the following case? (disassembly from CoreMark

   10248:       03012603                lw      a2,48(sp)
   1024c:       01c11583                lh      a1,28(sp)
   10250:       03412503                lw      a0,52(sp)

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In addition, consider the following scenario:

   10a84:       01c12083                lw      ra,28(sp)
   10a88:       07f47513                andi    a0,s0,127
   10a8c:       01812403                lw      s0,24(sp)
   10a90:       01412483                lw      s1,20(sp)
   10a94:       01012903                lw      s2,16(sp)
   10a98:       00c12983                lw      s3,12(sp)

It can be regarded as 5 lw. Roughly speaking, if peephole optimization can be applied, we shall benefit from further optimizations.

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Another case: (disassembly from CoreMark)

   10c08:       01162023                sw      a7,0(a2)
   10c0c:       00052783                lw      a5,0(a0)
   10c10:       00059883                lh      a7,0(a1)
   10c14:       00259603                lh      a2,2(a1)
   10c18:       00f82023                sw      a5,0(a6)
   10c1c:       01052023                sw      a6,0(a0)
   10c20:       00e82223                sw      a4,4(a6)
   10c24:       0006a783                lw      a5,0(a3)

Mixture of sw and lw.

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Another case: (disassembly from CoreMark)

   10c08:       01162023                sw      a7,0(a2)
   10c0c:       00052783                lw      a5,0(a0)
   10c10:       00059883                lh      a7,0(a1)
   10c14:       00259603                lh      a2,2(a1)
   10c18:       00f82023                sw      a5,0(a6)
   10c1c:       01052023                sw      a6,0(a0)
   10c20:       00e82223                sw      a4,4(a6)
   10c24:       0006a783                lw      a5,0(a3)

Mixture of sw and lw.

In this case, the memory address is not contiguous, what we can do just pack these instructions, but we cannot save any operation, such as checking misaligned.

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In addition, consider the following scenario:

   10a84:       01c12083                lw      ra,28(sp)
   10a88:       07f47513                andi    a0,s0,127
   10a8c:       01812403                lw      s0,24(sp)
   10a90:       01412483                lw      s1,20(sp)
   10a94:       01012903                lw      s2,16(sp)
   10a98:       00c12983                lw      s3,12(sp)

It can be regarded as 5 lw. Roughly speaking, if peephole optimization can be applied, we shall benefit from further optimizations.

In this case, we can pack the last four instruction lw. if we want to handle this case by packing 5 lw, we need to reorder the instruction. For example, swap the first and the second instruction.

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Can you handle the following case? (disassembly from CoreMark

   10248:       03012603                lw      a2,48(sp)
   1024c:       01c11583                lh      a1,28(sp)
   10250:       03412503                lw      a0,52(sp)

Ditto, if we want to handle this case, we need some strategies to reorder the instructions.

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In this pull request, let's concentrate on preliminary support of macro operation fusion. You shall add some comments for further efforts such as instruction reordering.

RVOP(fuse4, {
opcode_fuse_t *fuse = ir->fuse;
uint32_t addr = rv->X[fuse[0].rs1] + fuse[0].imm;
/* the memory addresses of the lw instructions are contiguous, so we only
* need to check the first lw instruction to determine if its memory address
* is misaligned or if the memory chunk does not exist.
*/
RV_EXC_MISALIGN_HANDLER(3, load, false, 1);
rv->X[fuse[0].rd] = rv->io.mem_read_w(rv, addr);
for (int i = 1; i < ir->imm2; i++) {
addr = rv->X[fuse[i].rs1] + fuse[i].imm;
rv->X[fuse[i].rd] = rv->io.mem_read_w(rv, addr);
}
})

static const void *dispatch_table[] = {
#define _(inst, can_branch) [rv_insn_##inst] = do_##inst,
RISCV_INSN_LIST
Expand Down Expand Up @@ -1337,7 +1386,6 @@ static void block_translate(riscv_t *rv, block_t *block)
/* compute the end of pc */
block->pc_end += ir->insn_len;
block->n_insn++;

/* stop on branch */
if (insn_is_branch(ir->opcode)) {
/* recursive jump translation */
Expand All @@ -1356,6 +1404,85 @@ static void block_translate(riscv_t *rv, block_t *block)
block->ir[block->n_insn - 1].tailcall = true;
}

#define COMBINE_MEM_OPS(RW) \
count = 1; \
next_ir = ir + 1; \
if (next_ir->opcode != IIF(RW)(rv_insn_lw, rv_insn_sw)) \
break; \
sign = (ir->imm - next_ir->imm) >> 31 ? -1 : 1; \
for (uint32_t j = 1; j < block->n_insn - 1 - i; j++) { \
next_ir = ir + j; \
if (next_ir->opcode != IIF(RW)(rv_insn_lw, rv_insn_sw) || \
ir->rs1 != next_ir->rs1 || ir->imm - next_ir->imm != 4 * sign) \
break; \
count++; \
} \
if (count > 1) { \
ir->opcode = IIF(RW)(rv_insn_fuse4, rv_insn_fuse3); \
ir->fuse = malloc(count * sizeof(opcode_fuse_t)); \
ir->imm2 = count; \
memcpy(ir->fuse, ir, sizeof(opcode_fuse_t)); \
ir->impl = dispatch_table[ir->opcode]; \
for (int j = 1; j < count; j++) { \
next_ir = ir + j; \
memcpy(ir->fuse + j, next_ir, sizeof(opcode_fuse_t)); \
next_ir->opcode = rv_insn_nop; \
next_ir->impl = dispatch_table[next_ir->opcode]; \
} \
}


/* examine whether instructions in a block match a specific pattern. If so,
* rewrite them into fused instructions.
*
* We plan to devise strategies to increase the number of instructions that
* match the pattern, such as reordering the instructions.
*/
static void match_pattern(block_t *block)
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{
for (uint32_t i = 0; i < block->n_insn - 1; i++) {
rv_insn_t *ir = block->ir + i, *next_ir = NULL;
int32_t count = 0, sign = 1;
switch (ir->opcode) {
case rv_insn_auipc:
next_ir = ir + 1;
if (next_ir->opcode == rv_insn_addi && ir->rd == next_ir->rs1) {
/* the destination register of instruction auipc is equal to the
* source register 1 of next instruction addi */
ir->opcode = rv_insn_fuse1;
ir->rd = next_ir->rd;
ir->imm2 = next_ir->imm;
ir->impl = dispatch_table[ir->opcode];
next_ir->opcode = rv_insn_nop;
next_ir->impl = dispatch_table[next_ir->opcode];
} else if (next_ir->opcode == rv_insn_add &&
ir->rd == next_ir->rs2) {
/* the destination register of instruction auipc is equal to the
* source register 2 of next instruction add */
ir->opcode = rv_insn_fuse2;
ir->rd = next_ir->rd;
ir->rs1 = next_ir->rs1;
ir->impl = dispatch_table[ir->opcode];
next_ir->opcode = rv_insn_nop;
next_ir->impl = dispatch_table[next_ir->opcode];
}
break;
/* If the memory addresses of a sequence of store or load instructions
* are contiguous, combine these instructions.
*/
case rv_insn_sw:
COMBINE_MEM_OPS(0);
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break;
case rv_insn_lw:
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COMBINE_MEM_OPS(1);
break;
/* FIXME: lui + addi */
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Code scanning / CodeQL

FIXME comment

FIXME comment: lui + addi
/* TODO: mixture of sw and lw */
/* TODO: reorder insturction to match pattern */
}
}
}

static block_t *prev = NULL;
static block_t *block_find_or_translate(riscv_t *rv)
{
Expand All @@ -1374,6 +1501,12 @@ static block_t *block_find_or_translate(riscv_t *rv)

/* translate the basic block */
block_translate(rv, next);
#if RV32_HAS(GDBSTUB)
if (!rv->debug_mode)
#endif
/* macro operation fusion */
match_pattern(next);


/* insert the block into block map */
block_insert(&rv->block_map, next);
Expand Down
2 changes: 2 additions & 0 deletions src/riscv.c
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Expand Up @@ -25,6 +25,8 @@ void block_map_clear(block_map_t *map)
block_t *block = map->map[i];
if (!block)
continue;
for (uint32_t i = 0; i < block->n_insn; i++)
free(block->ir[i].fuse);
free(block->ir);
free(block);
map->map[i] = NULL;
Expand Down