[PATCH][RFA][PR tree-optimization/79095] Improve overflow test optimization and avoid invalid warnings

[Marc Glisse]

On Wed, 25 Jan 2017, Jeff Law wrote:

> As has been discussed extensively, we're not doing a good job at simplifying 
> overflow tests, particularly those which collapse down to an EQ/NE test.
>
> x + -1 > x  -> x == 0
> x + -1 < x  -> x != 0
> x + 1 < x   -> x == -1U
> x + 1 > x   -> x != -1U
>
> The simplifications allow us to propagate a constant for X into one ARM of 
> the associated IF/ELSE construct.  For C++ std::vector operations those 
> propagations can eliminate lots of unnecessary code.
>
> Those propagations also eliminate (by way of removing unnecessary code) false 
> positive warnings for memset calls that come from std::vector operations.
>
> This patch does two things.
>
> 1. It adds special case patterns to the A+CST CMP A pattern for cases where 
> CST is 1 or -1 where the result turns into A EQ/NE 0 or A EQ/NE -1U.  These 
> special patterns are applied regardless of the single_use status of the 
> expression.
>
> 2. It adds a call to fold_stmt in simplify_cond_using_ranges.  This allows 
> VRP to transform the code early and the first DOM pass to often see the 
> simpified conditional and thus optimize better, rather than waiting for 
> forwprop3 to simplify the conditional and the last DOM pass to optimize the 
> code.
>
> Bootstrapped and regression tested on x86_64-linux-gnu.  OK for the trunk?

I assume this causes a regression for code like

unsigned f(unsigned a){
   unsigned b=a+1;
   if(b<a)return 42;
   return b;
}

? On the other hand, the optimization is already very fragile, if I write 
b<=a (which is equivalent since 1 != 0), it doesn't apply.

We currently get
 	addl	$1, %edi
 	movl	$42, %eax
 	cmovnc	%edi, %eax
or almost as good with b==0
 	movl	%edi, %eax
 	movl	$42, %edx
 	addl	$1, %eax
 	cmove	%edx, %eax
while with a==-1 we have the redundant comparison
 	leal	1(%rdi), %eax
 	cmpl	$-1, %edi
 	movl	$42, %edx
 	cmove	%edx, %eax

Simplifying x + 1 < x to x + 1 == 0 might not be enough to simplify your 
examples though I guess?

-- 
Marc Glisse