signed/unsigned comparison

.gitignore

*.o
main

README.md

Run demo

make && ./main

Explaination

见cmpui.S

cmpui.c

#include <inttypes.h>

int
lt_u64_i64(uint64_t u, int64_t i) {
    return u < i;
}

int
lt_i64_u64(int64_t i, uint64_t u) {
    return i < u;
}

int
lt_i64_i64(int64_t i1, int64_t i2) {
    return i1 < i2;
}

int
lt_u64_u64(uint64_t u1, uint64_t u2) {
    return u1 < u2;
}

cmpui.S

	.file	"cmpui.c"
	.text
	.p2align 4,,15
	.globl	lt_u64_i64
	.type	lt_u64_i64, @function
lt_u64_i64:
.LFB4:
	.cfi_startproc
	xorl	%eax, %eax
	cmpq	%rdi, %rsi
	seta	%al
	ret
	.cfi_endproc
.LFE4:
	.size	lt_u64_i64, .-lt_u64_i64
	.p2align 4,,15
	.globl	lt_i64_u64
	.type	lt_i64_u64, @function
lt_i64_u64:
.LFB5:
	.cfi_startproc
	xorl	%eax, %eax
	cmpq	%rsi, %rdi
	setb	%al
	ret
	.cfi_endproc
.LFE5:
	.size	lt_i64_u64, .-lt_i64_u64
	.p2align 4,,15
	.globl	lt_i64_i64
	.type	lt_i64_i64, @function
lt_i64_i64:
.LFB6:
	.cfi_startproc
	xorl	%eax, %eax
	cmpq	%rsi, %rdi
	setl	%al
	ret
	.cfi_endproc
.LFE6:
	.size	lt_i64_i64, .-lt_i64_i64
	.p2align 4,,15
	.globl	lt_u64_u64
	.type	lt_u64_u64, @function
lt_u64_u64:
.LFB7:
	.cfi_startproc
	xorl	%eax, %eax
	cmpq	%rsi, %rdi
	setb	%al
	ret
	.cfi_endproc
.LFE7:
	.size	lt_u64_u64, .-lt_u64_u64
	.ident	"GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
	.section	.note.GNU-stack,"",@progbits

main.c

#include <stdio.h>
#include <inttypes.h>

extern int lt_u64_i64(uint64_t u, int64_t i);
extern int lt_i64_u64(int64_t i, uint64_t u);
extern int lt_i64_i64(int64_t i1, int64_t i2);
extern int lt_u64_u64(uint64_t u1, uint64_t u2);

#define PRN_BIN_OP(func, lhs, rhs, fmt) \
    printf(#func "(" #lhs "," #rhs ") = " fmt "\n", func(lhs, rhs))

int
main() {
    PRN_BIN_OP(lt_u64_i64, 123, -123, "%d");
    PRN_BIN_OP(lt_i64_u64, -1, 123, "%d");
    PRN_BIN_OP(lt_i64_i64, -1, -2, "%d");
    PRN_BIN_OP(lt_u64_u64, 1, 2, "%d");
    return 0;
}

Makefile

all : main cmpui.S

main : main.o cmpui.o

%.S : %.c
	$(CC) -c -S $< -O2 -o $@

在cmpui.S:11:lt_u64_i64，这里用的是seta，即为set above，是unsigned comparison。

在cmpui.S:37:lt_i64_i64，这里用的是setl，即为set less than，是signed comparison。

更多的解释见c-faq。

老衲要嘿嘿了