Blink executes FDIV as FDIVR and FDIVR as FDIV
Author: alxrmorozovCreated Jun 13, 2025Updated Jun 13, 2025
This test succeeds on real CPU but fails in blink:
#include "test/asm/mac.inc"
.globl _start
_start:
// make -j8 o//blink o//test/asm/fdiv.elf
// o//blink/blinkenlights o//test/asm/fdiv.elf
.test "fdiv"
fld1
fldz
fdiv %st,%st(1)
fistpq -8(%rsp)
fistpq -8(%rsp)
cmpq $0,-8(%rsp)
.e
.test "fdivr"
fldz
fld1
fdivr %st,%st(1)
fistpq -8(%rsp)
fistpq -8(%rsp)
cmpq $0,-8(%rsp)
.e
"test succeeded":
.exitIf I replace fdiv with fdivr and fdivr with fdiv it succeeds in blink. It looks like this is connected with this AT&T assembler feature: https://sourceware.org/binutils/docs/as.html#i386_002dBugs But FSUB and FSUBR work correctly.
Source: jart/blink