On Wed, Dec 16, 2015 at 9:11 AM, Thomas Preud'homme
<thomas.preudho...@foss.arm.com> wrote:
> During reorg pass, thumb1_reorg () is tasked with rewriting mov rd, rn to
> subs rd, rn, 0 to avoid a comparison against 0 instruction before doing a
> conditional branch based on it. The actual avoiding of cmp is done in
> cbranchsi4_insn instruction C output template. When the condition is met, the
> source register (rn) is also propagated into the comparison in place the
> destination register (rd).
>
> However, right now thumb1_reorg () only look for a mov followed by a
> cbranchsi but does not check whether the comparison in cbranchsi is against
> the constant 0. This is not safe because a non clobbering instruction could
> exist between the mov and the comparison that modifies the source register.
> This is what happens here with a post increment of the source register after
> the mov, which skip the &a[i] == &a[1] comparison for iteration i == 1.
>
> This patch fixes the issue by checking that the comparison is against
> constant 0.
>
> ChangeLog entry is as follow:
>
>
> *** gcc/ChangeLog ***
>
> 2015-12-07 Thomas Preud'homme <thomas.preudho...@arm.com>
>
> * config/arm/arm.c (thumb1_reorg): Check that the comparison is
> against the constant 0.
>
OK.
Ramana
>
> diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
> index 42bf272..49c0a06 100644
> --- a/gcc/config/arm/arm.c
> +++ b/gcc/config/arm/arm.c
> @@ -17195,7 +17195,7 @@ thumb1_reorg (void)
> FOR_EACH_BB_FN (bb, cfun)
> {
> rtx dest, src;
> - rtx pat, op0, set = NULL;
> + rtx cmp, op0, op1, set = NULL;
> rtx_insn *prev, *insn = BB_END (bb);
> bool insn_clobbered = false;
>
> @@ -17208,8 +17208,13 @@ thumb1_reorg (void)
> continue;
>
> /* Get the register with which we are comparing. */
> - pat = PATTERN (insn);
> - op0 = XEXP (XEXP (SET_SRC (pat), 0), 0);
> + cmp = XEXP (SET_SRC (PATTERN (insn)), 0);
> + op0 = XEXP (cmp, 0);
> + op1 = XEXP (cmp, 1);
> +
> + /* Check that comparison is against ZERO. */
> + if (!CONST_INT_P (op1) || INTVAL (op1) != 0)
> + continue;
>
> /* Find the first flag setting insn before INSN in basic block BB. */
> gcc_assert (insn != BB_HEAD (bb));
> @@ -17249,7 +17254,7 @@ thumb1_reorg (void)
> PATTERN (prev) = gen_rtx_SET (dest, src);
> INSN_CODE (prev) = -1;
> /* Set test register in INSN to dest. */
> - XEXP (XEXP (SET_SRC (pat), 0), 0) = copy_rtx (dest);
> + XEXP (cmp, 0) = copy_rtx (dest);
> INSN_CODE (insn) = -1;
> }
> }
>
>
> Testsuite shows no regression when run for arm-none-eabi with -mcpu=cortex-m0
> -mthumb
>
> Is this ok for trunk?
>
> Best regards,
>
> Thomas
>
