Re: [PATCH 0/3] Add __builtin_load_no_speculate

[Bernd Edlinger <bernd dot edlinger at hotmail dot de>]

I thought about your new builtin again, and I wonder if
something like that might work as well?


cat despec.s
	.arch armv7-a
	.eabi_attribute 28, 1
	.eabi_attribute 20, 1
	.eabi_attribute 21, 1
	.eabi_attribute 23, 3
	.eabi_attribute 24, 1
	.eabi_attribute 25, 1
	.eabi_attribute 26, 2
	.eabi_attribute 30, 4
	.eabi_attribute 34, 1
	.eabi_attribute 18, 4
	.section	.text.startup,"ax",%progbits
	.align	2
	.global	despec_i
	.syntax unified
	.arm
	.fpu vfpv3-d16

despec_i:
	cmp r0,#0
	beq L0
	ldr r0,[r1]
	moveq r0,r2
	nop {0x14} @ CSDB
	str r0,[r1]
	mov r0,#1
	bx lr
L0:	mov r0,#0
	bx lr

cat test.c
extern int despec_i(int predicate, int *untrusted, int fallback);
#define N 8
int a[N] = {1,2,3,4,5,7,8,9};
int a2[0x200];
int test(int untrust)
{
   int x = 0;
   if (despec_i(untrust >= 0 && untrust < N, &untrust, 0))
   {
      int v = a[untrust] & 0x1 ? 0x100 : 0x0;
      x = a2[v];
   }
   return x;
}


So this should feed the predicate through the builtin, and
clear the untrusted value when the condition has been been
mis-predicted.

Wouldn't that be more flexible to use?
Or am I missing something?


As a side note: I noticed that "nop {0x14}" seems to produce the correct
assembler opcode without the need for using a .insn code.


Bernd.