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6911 - comparing floats 

It turns out floating-point operations set different flags than most instructions.
We have to branch on them using unsigned jumps.

https://stackoverflow.com/questions/7057501/x86-assembler-floating-point-compare/7057771#7057771
This commit is contained in:
Kartik Agaram 2020-09-30 22:50:26 -07:00
parent d564633b24
commit 656b840e7f
5 changed files with 63 additions and 23 deletions
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12
014indirect_addressing.cc
View File

@ -538,7 +538,7 @@ void test_compare_mem_at_r32_with_r32_equal() {
put_new(Name, "3b", "compare: set SF if r32 < rm32 (cmp)");
:(code)
void test_compare_r32_with_mem_at_r32_greater() {
void test_compare_r32_with_mem_at_rm32_greater() {
Reg[EAX].i = 0x2000;
Reg[EBX].i = 0x0a0b0c0d;
run(
@ -576,7 +576,7 @@ case 0x3b: { // set SF if r32 < r/m32
}
:(code)
void test_compare_r32_with_mem_at_r32_lesser_unsigned_and_signed() {
void test_compare_r32_with_mem_at_rm32_lesser_unsigned_and_signed() {
Reg[EAX].i = 0x2000;
Reg[EBX].i = 0x0a0b0c07;
run(
@ -595,7 +595,7 @@ void test_compare_r32_with_mem_at_r32_lesser_unsigned_and_signed() {
);
}
void test_compare_r32_with_mem_at_r32_lesser_unsigned_and_signed_due_to_overflow() {
void test_compare_r32_with_mem_at_rm32_lesser_unsigned_and_signed_due_to_overflow() {
Reg[EAX].i = 0x2000;
Reg[EBX].i = 0x7fffffff; // largest positive signed integer
run(
@ -614,7 +614,7 @@ void test_compare_r32_with_mem_at_r32_lesser_unsigned_and_signed_due_to_overflow
);
}
void test_compare_r32_with_mem_at_r32_lesser_signed() {
void test_compare_r32_with_mem_at_rm32_lesser_signed() {
Reg[EAX].i = 0x2000;
Reg[EBX].i = 0xffffffff; // -1
run(
@ -633,7 +633,7 @@ void test_compare_r32_with_mem_at_r32_lesser_signed() {
);
}
void test_compare_r32_with_mem_at_r32_lesser_unsigned() {
void test_compare_r32_with_mem_at_rm32_lesser_unsigned() {
Reg[EAX].i = 0x2000;
Reg[EBX].i = 0x00000001; // 1
run(
@ -652,7 +652,7 @@ void test_compare_r32_with_mem_at_r32_lesser_unsigned() {
);
}
void test_compare_r32_with_mem_at_r32_equal() {
void test_compare_r32_with_mem_at_rm32_equal() {
Reg[EAX].i = 0x2000;
Reg[EBX].i = 0x0a0b0c0d;
run(

16
017jump_disp8.cc
View File

@ -135,8 +135,8 @@ void test_jne_disp8_fail() {
//:: jump if greater
:(before "End Initialize Op Names")
put_new(Name, "7f", "jump disp8 bytes away if greater (signed), if ZF is unset and SF == OF (jcc/jg/jnle)");
put_new(Name, "77", "jump disp8 bytes away if greater (unsigned), if ZF is unset and CF is unset (jcc/ja/jnbe)");
put_new(Name, "7f", "jump disp8 bytes away if greater, if ZF is unset and SF == OF (jcc/jg/jnle)");
put_new(Name, "77", "jump disp8 bytes away if greater (addr, float), if ZF is unset and CF is unset (jcc/ja/jnbe)");
:(code)
void test_jg_disp8_success() {
@ -199,8 +199,8 @@ void test_jg_disp8_fail() {
//:: jump if greater or equal
:(before "End Initialize Op Names")
put_new(Name, "7d", "jump disp8 bytes away if greater or equal (signed), if SF == OF (jcc/jge/jnl)");
put_new(Name, "73", "jump disp8 bytes away if greater or equal (unsigned), if CF is unset (jcc/jae/jnb)");
put_new(Name, "7d", "jump disp8 bytes away if greater or equal, if SF == OF (jcc/jge/jnl)");
put_new(Name, "73", "jump disp8 bytes away if greater or equal (addr, float), if CF is unset (jcc/jae/jnb)");
:(code)
void test_jge_disp8_success() {
@ -261,8 +261,8 @@ void test_jge_disp8_fail() {
//:: jump if lesser
:(before "End Initialize Op Names")
put_new(Name, "7c", "jump disp8 bytes away if lesser (signed), if SF != OF (jcc/jl/jnge)");
put_new(Name, "72", "jump disp8 bytes away if lesser (unsigned), if CF is set (jcc/jb/jnae)");
put_new(Name, "7c", "jump disp8 bytes away if lesser, if SF != OF (jcc/jl/jnge)");
put_new(Name, "72", "jump disp8 bytes away if lesser (addr, float), if CF is set (jcc/jb/jnae)");
:(code)
void test_jl_disp8_success() {
@ -325,8 +325,8 @@ void test_jl_disp8_fail() {
//:: jump if lesser or equal
:(before "End Initialize Op Names")
put_new(Name, "7e", "jump disp8 bytes away if lesser or equal (signed), if ZF is set or SF != OF (jcc/jle/jng)");
put_new(Name, "76", "jump disp8 bytes away if lesser or equal (unsigned), if ZF is set or CF is set (jcc/jbe/jna)");
put_new(Name, "7e", "jump disp8 bytes away if lesser or equal, if ZF is set or SF != OF (jcc/jle/jng)");
put_new(Name, "76", "jump disp8 bytes away if lesser or equal (addr, float), if ZF is set or CF is set (jcc/jbe/jna)");
:(code)
void test_jle_disp8_equal() {

16
018jump_disp32.cc
View File

@ -135,8 +135,8 @@ void test_jne_disp32_fail() {
//:: jump if greater
:(before "End Initialize Op Names")
put_new(Name_0f, "8f", "jump disp32 bytes away if greater (signed), if ZF is unset and SF == OF (jcc/jg/jnle)");
put_new(Name_0f, "87", "jump disp32 bytes away if greater (unsigned), if ZF is unset and CF is unset (jcc/ja/jnbe)");
put_new(Name_0f, "8f", "jump disp32 bytes away if greater, if ZF is unset and SF == OF (jcc/jg/jnle)");
put_new(Name_0f, "87", "jump disp32 bytes away if greater (addr, float), if ZF is unset and CF is unset (jcc/ja/jnbe)");
:(code)
void test_jg_disp32_success() {
@ -199,8 +199,8 @@ void test_jg_disp32_fail() {
//:: jump if greater or equal
:(before "End Initialize Op Names")
put_new(Name_0f, "8d", "jump disp32 bytes away if greater or equal (signed), if SF == OF (jcc/jge/jnl)");
put_new(Name_0f, "83", "jump disp32 bytes away if greater or equal (unsigned), if CF is unset (jcc/jae/jnb)");
put_new(Name_0f, "8d", "jump disp32 bytes away if greater or equal, if SF == OF (jcc/jge/jnl)");
put_new(Name_0f, "83", "jump disp32 bytes away if greater or equal (addr, float), if CF is unset (jcc/jae/jnb)");
:(code)
void test_jge_disp32_success() {
@ -261,8 +261,8 @@ void test_jge_disp32_fail() {
//:: jump if lesser
:(before "End Initialize Op Names")
put_new(Name_0f, "8c", "jump disp32 bytes away if lesser (signed), if SF != OF (jcc/jl/jnge)");
put_new(Name_0f, "82", "jump disp32 bytes away if lesser (unsigned), if CF is set (jcc/jb/jnae)");
put_new(Name_0f, "8c", "jump disp32 bytes away if lesser, if SF != OF (jcc/jl/jnge)");
put_new(Name_0f, "82", "jump disp32 bytes away if lesser (addr, float), if CF is set (jcc/jb/jnae)");
:(code)
void test_jl_disp32_success() {
@ -325,8 +325,8 @@ void test_jl_disp32_fail() {
//:: jump if lesser or equal
:(before "End Initialize Op Names")
put_new(Name_0f, "8e", "jump disp32 bytes away if lesser or equal (signed), if ZF is set or SF != OF (jcc/jle/jng)");
put_new(Name_0f, "86", "jump disp32 bytes away if lesser or equal (unsigned), if ZF is set or CF is set (jcc/jbe/jna)");
put_new(Name_0f, "8e", "jump disp32 bytes away if lesser or equal, if ZF is set or SF != OF (jcc/jle/jng)");
put_new(Name_0f, "86", "jump disp32 bytes away if lesser or equal (addr, float), if ZF is set or CF is set (jcc/jbe/jna)");
:(code)
void test_jle_disp32_equal() {

39
023float.cc
View File

@ -375,3 +375,42 @@ float* effective_address_float(uint8_t modrm) {
trace(Callstack_depth+1, "run") << "effective address contains " << read_mem_f32(addr) << end();
return mem_addr_f32(addr);
}
//: compare
:(before "End Initialize Op Names")
put_new(Name_0f, "2f", "compare: set SF if x32 < xm32 (comiss)");
:(code)
void test_compare_x32_with_mem_at_rm32() {
Reg[EAX].i = 0x2000;
Xmm[3] = 0.5;
run(
"== code 0x1\n"
// op ModR/M SIB displacement immediate
" 0f 2f 18 \n" // compare XMM3 with *EAX
// ModR/M in binary: 00 (indirect mode) 011 (lhs XMM3) 000 (rhs EAX)
"== data 0x2000\n"
"00 00 00 00\n" // 0x00000000 = 0.0
);
CHECK_TRACE_CONTENTS(
"run: compare XMM3 with x/m32\n"
"run: effective address is 0x00002000 (EAX)\n"
"run: SF=0; ZF=0; CF=0; OF=0\n"
);
}
:(before "End Two-Byte Opcodes Starting With 0f")
case 0x2f: { // set SF if x32 < x/m32
const uint8_t modrm = next();
const uint8_t reg1 = (modrm>>3)&0x7;
trace(Callstack_depth+1, "run") << "compare " << Xname[reg1] << " with x/m32" << end();
const float* arg2 = effective_address_float(modrm);
// Flag settings carefully copied from the Intel manual.
// See also https://stackoverflow.com/questions/7057501/x86-assembler-floating-point-compare/7057771#7057771
SF = ZF = CF = OF = false;
if (Xmm[reg1] == *arg2) ZF = true;
if (Xmm[reg1] < *arg2) CF = true;
trace(Callstack_depth+1, "run") << "SF=" << SF << "; ZF=" << ZF << "; CF=" << CF << "; OF=" << OF << end();
break;
}

3
033check_operands.cc
View File

@ -677,7 +677,8 @@ put_new(Permitted_arguments_0f, "8f", 0x10);
//// Class M: using ModR/M byte
// imm32 imm8 disp32 |disp16 disp8 subop modrm
// 0 0 0 |0 0 0 1
put_new(Permitted_arguments_0f, "af", 0x01);
put_new(Permitted_arguments_0f, "2f", 0x01); // compare floats
put_new(Permitted_arguments_0f, "af", 0x01); // multiply ints
// setcc
put_new(Permitted_arguments_0f, "92", 0x01);
put_new(Permitted_arguments_0f, "93", 0x01);