ESP32 Unaligned access compare disassembly

Makefile

.PHONY: all

XTENSA_PREFIX ?= xtensa-esp32-elf-
XTENSA_BIN ?= $(HOME)/xtensa-esp32-elf/1.22.0-80/bin
CC := $(XTENSA_BIN)/$(XTENSA_PREFIX)gcc
CXX := $(XTENSA_BIN)/$(XTENSA_PREFIX)g++
CC := $(XTENSA_BIN)/$(XTENSA_PREFIX)ld
OBJDUMP := $(XTENSA_BIN)/$(XTENSA_PREFIX)objdump

CXXFLAGS := -g -Os -std=c++11

all: test.lst
clean:
	-$(RM) test.lst test.elf test.o

%.lst: %.elf
	$(OBJDUMP) -dSC $< > $@

test.elf: test.o
	$(CXX) -g -nostdlib -nostartfiles -o $@ $^

%.o: %.cpp
	$(CXX) -c $(CXXFLAGS) -o $@ $<

test.cpp

#include <cstdint>

static volatile std::uint8_t b[10] = {0x11, 0x22, 0xff, 0xd9, 0xaa, 0xde, 0xad, 0xbe, 0xef, 0xca};

bool __attribute__((noinline)) unaligned_compare_ub()
{
    std::uint32_t sch = *((uint32_t*)&b[2]) & 0xffffff;
    return sch == 0xaad9ff;
}

bool __attribute__((noinline)) unaligned_compare_ok()
{
    return b[2] == 0xff && b[3] == 0xd9 && b[4] == 0xaa;
}


static volatile bool result_ub;
static volatile bool result_ok;

int main()
{
    result_ub = unaligned_compare_ub();
    result_ok = unaligned_compare_ok();
}

test.lst

test.elf:     file format elf32-xtensa-le


Disassembly of section .text:

00400074 <unaligned_compare_ub()-0x10>:
  400074:	400107        	bnone	a1, a0, 4000b8 <unaligned_compare_ok()+0x8>
  400077:	260100        	lsi	f0, a1, 152
  40007a:	12ff55        	call4	413070 <_end+0x12f5c>
  40007d:	004001        	l32r	a0, 3c0180 <unaligned_compare_ub()-0x3ff04>
  400080:	400111        	l32r	a1, 3d0084 <unaligned_compare_ub()-0x30000>
	...

00400084 <unaligned_compare_ub()>:
#include <cstdint>

static volatile std::uint8_t b[10] = {0x11, 0x22, 0xff, 0xd9, 0xaa, 0xde, 0xad, 0xbe, 0xef, 0xca};

bool __attribute__((noinline)) unaligned_compare_ub()
{
  400084:	004136        	entry	a1, 32
    std::uint32_t sch = *((uint32_t*)&b[2]) & 0xffffff;
  400087:	fffb21        	l32r	a2, 400074 <unaligned_compare_ub()-0x10>
  40008a:	822b      	addi.n	a8, a2, 2
  40008c:	0202a2        	l8ui	a10, a2, 2
  40008f:	030222        	l8ui	a2, a2, 3
  400092:	112280        	slli	a2, a2, 8
  400095:	2092a0        	or	a9, a2, a10
  400098:	020822        	l8ui	a2, a8, 2
    return sch == 0xaad9ff;
  40009b:	fff781        	l32r	a8, 400078 <unaligned_compare_ub()-0xc>

static volatile std::uint8_t b[10] = {0x11, 0x22, 0xff, 0xd9, 0xaa, 0xde, 0xad, 0xbe, 0xef, 0xca};

bool __attribute__((noinline)) unaligned_compare_ub()
{
    std::uint32_t sch = *((uint32_t*)&b[2]) & 0xffffff;
  40009e:	112200        	slli	a2, a2, 16
    return sch == 0xaad9ff;
  4000a1:	202290        	or	a2, a2, a9
  4000a4:	828a      	add.n	a8, a2, a8
  4000a6:	190c      	movi.n	a9, 1
  4000a8:	020c      	movi.n	a2, 0
  4000aa:	832980        	moveqz	a2, a9, a8
}
  4000ad:	f01d      	retw.n
	...

004000b0 <unaligned_compare_ok()>:

bool __attribute__((noinline)) unaligned_compare_ok()
{
  4000b0:	004136        	entry	a1, 32
    return b[2] == 0xff && b[3] == 0xd9 && b[4] == 0xaa;
  4000b3:	fff081        	l32r	a8, 400074 <unaligned_compare_ub()-0x10>
  4000b6:	ffa0a2        	movi	a10, 255
  4000b9:	020892        	l8ui	a9, a8, 2
  4000bc:	020c      	movi.n	a2, 0
  4000be:	749090        	extui	a9, a9, 0, 8
  4000c1:	1f99a7        	bne	a9, a10, 4000e4 <unaligned_compare_ok()+0x34>
  4000c4:	030892        	l8ui	a9, a8, 3
  4000c7:	d9a0a2        	movi	a10, 217
  4000ca:	749090        	extui	a9, a9, 0, 8
  4000cd:	1399a7        	bne	a9, a10, 4000e4 <unaligned_compare_ok()+0x34>
  4000d0:	040882        	l8ui	a8, a8, 4
  4000d3:	749080        	extui	a9, a8, 0, 8
  4000d6:	56af82        	movi	a8, -170
  4000d9:	898a      	add.n	a8, a9, a8
  4000db:	01a092        	movi	a9, 1
  4000de:	832980        	moveqz	a2, a9, a8
  4000e1:	742020        	extui	a2, a2, 0, 8
}
  4000e4:	f01d      	retw.n
	...

004000e8 <main>:

static volatile bool result_ub;
static volatile bool result_ok;

int main()
{
  4000e8:	004136        	entry	a1, 32
    result_ub = unaligned_compare_ub();
  4000eb:	fff9a5        	call8	400084 <unaligned_compare_ub()>
  4000ee:	ffe381        	l32r	a8, 40007c <unaligned_compare_ub()-0x8>
    result_ok = unaligned_compare_ok();
}
  4000f1:	020c      	movi.n	a2, 0
static volatile bool result_ub;
static volatile bool result_ok;

int main()
{
    result_ub = unaligned_compare_ub();
  4000f3:	0020c0        	memw
  4000f6:	0048a2        	s8i	a10, a8, 0
    result_ok = unaligned_compare_ok();
  4000f9:	fffb65        	call8	4000b0 <unaligned_compare_ok()>
  4000fc:	ffe181        	l32r	a8, 400080 <unaligned_compare_ub()-0x4>
  4000ff:	0020c0        	memw
  400102:	0048a2        	s8i	a10, a8, 0
}
  400105:	f01d      	retw.n