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Re: [PATCH v4] PR target/30315 i386 missed optimization detecting overflows
On Tue, Aug 14, 2007 at 03:34:53PM +0200, Paolo Bonzini wrote:
>
> >+/* Canonicalize overflow checks to save on the insn patterns. We change
> >+ "a + b < b" into "a + b < a" and "a + b >= b" into "a + b >= a". */
> >+#define CANONICALIZE_COMPARISON(code, op0, op1) \
> >+{ \
> >+ if ((code == LTU || code == GEU) \
> >+ && GET_CODE (op0) == PLUS \
> >+ && rtx_equal_p (op1, XEXP (op0, 1))) \
> >+ op1 = XEXP (op0, 0); \
> >+}
>
> Sorry for complicating your useful work.
>
> It looks to me like this canonicalization belongs in simplify-rtx.c. Of
> course, this can be done in a follow-up patch.
I agree. We also want to do something at the tree level so
sum = a + b;
if (sum < a)
abort ();
if (sum < b)
abort ();
is optimized into
sum = a + b;
if (sum < a)
abort ();
--
Rask Ingemann Lambertsen