# [llvm-dev] [RFC] Value Range Based Optimization Opportunity in LLVM

Tony Jiang via llvm-dev llvm-dev at lists.llvm.org
Thu Aug 31 13:54:25 PDT 2017

Hi All,

We have recently found some optimization opportunities created by
replicating code into branches in order to enable optimization. In
general, the optimization opportunity we are pursuing is like the
following.

Given pseudo-code:

// block A
if (some condition)
// block B
// block C

If it can be efficiently proven that some portion of block C can be
simplified had control flow not gone through the if statement, it might be
useful to convert this CFG into a diamond and hoist that portion of block
C into both block B and the new block.

Consider the following example:

int test(int *Ptr, int a, int b, int c, int d) {
int Ret = 0;
if (__builtin_expect(!Ptr, 0)) {
Ret = calli(a);
// return Ret / (a|1) / (b|1) / (c|1) / (d|1); // Copy return to here
}
return Ret / (a|1) / (b|1) / (c|1) / (d|1); // This can be simplified to
return 0
}

In this case, replicating the return statement in the branch allows the
optimizer to prove the value of Ret at the end of the function is 0 and
eliminate the arithmetic calculations.

A second example:

unsigned long funcReturningArbitraryi64(unsigned char *p);
#define LENGTH(uv)  ( (uv) < 0x80             ? 1 :  \
(uv) < 0x800            ? 2 :  \
(uv) < 0x10000          ? 3 :  \
(uv) < 0x200000         ? 4 :  \
(uv) < 0x4000000        ? 5 :  \
(uv) < 0x80000000       ? 6 : 7 )

int func(unsigned char *p, bool flag)
{
unsigned long c = *p;
int len;
// ...
#ifdef _ORIG
if(flag) {
// ...
c = funcReturningArbitraryi64(p);
}
len = LENGTH(c);
#else
if(flag) {
// ...
c = funcReturningArbitraryi64(p);
len = LENGTH(c);
} else {
len = LENGTH(c);
}
#endif

// ...

return len;
}

In this case, we see that creating an else block and replicating the
return statement in both the if and else branch (like the code snippet
guarded by the #else) enables the macro UNISKIP in the else branch to be
optimized.

Most of the examples we have come up with so far are centered around value
ranges along the conditional branches. When the range of values a symbol
can have along different branches is provably different, opportunities for
optimization may arise. However, this likely isn't the only category of
optimizations that could benefit from this.

Is there an existing transformation in LLVM that should be doing this
already that is missing this opportunity? If not, we would like to pursue
adding this. Of course, this optimization would be subject to a cost model
as it may result in code growth. For example, it may not be advantageous
to do this if the simplified branch is cold. If anyone has any
comments/suggestions we are very much interested in hearing them.

Regards,

Tony Jiang, M.Sc.
LLVM PPC Backend Development
IBM Toronto Lab, C2/712/8200/MKM
8200 Warden Ave, Markham, L6G 1C7
Email: jtony at ca.ibm.com
Phone: 905-413-3676

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