Given the following SCEV,
(sext i32 {2,+,1}<%for.body> to i64)
from the following C source,
void strong3(int *A, int *B, int n) {
for (int i = 0; i < n; i++) {
A[i + 2] = i;
…
}
}
Since the No-Wrap flag is set on the addrec, can’t I safely rewrite it as
{2,+,1}<%for.body>
If I can, why isn’t the SCEV package simplifying things for me?
Thanks,
Preston
The short answer is that SCEV is terrible at preserving NSW flags. I personally don’t believe they belong in SCEV but the merits of making any design change here are dubious.
To understand one example of SCEV dropping NSW, see createSCEV for Instruction::Add. Synopsis: your add is not a “basic induction variable” so its NSW flag does not bound the number of loop iterations. We only know that the add’s original IR users expect NSW. There could be other IR adds with the same expression, but without the NSW flag. SCEV doesn’t know anything about acyclic control flow or IR users, so it must drop the flags.
I would try hard not to rely on NSW flags on arbitrary SCEVs. I would first find the phi or basic induction variable before checking the recurrence’s NSW flag. Or, better yet, only rely on SCEVAddRec’s NW (no self-wrap flag) rather than NSW. Notice that the NW is preserved in your add’s recurrence!
-Andy
Given the following SCEV,
(sext i32 {2,+,1}<%for.body> to i64)
from the following C source,
void strong3(int *A, int *B, int n) {
for (int i = 0; i < n; i++) {
A[i + 2] = i;…
}
}Since the No-Wrap flag is set on the addrec, can’t I safely rewrite it as
{2,+,1}<%for.body>
If I can, why isn’t the SCEV package simplifying things for me?
The short answer is that SCEV is terrible at preserving NSW flags. I personally don’t believe they belong in SCEV but the merits of making any design change here are dubious.
To understand one example of SCEV dropping NSW, see createSCEV for Instruction::Add. Synopsis: your add is not a “basic induction variable” so its NSW flag does not bound the number of loop iterations. We only know that the add’s original IR users expect NSW. There could be other IR adds with the same expression, but without the NSW flag. SCEV doesn’t know anything about acyclic control flow or IR users, so it must drop the flags.
I would try hard not to rely on NSW flags on arbitrary SCEVs. I would first find the phi or basic induction variable before checking the recurrence’s NSW flag. Or, better yet, only rely on SCEVAddRec’s NW (no self-wrap flag) rather than NSW. Notice that the NW is preserved in your add’s recurrence!
-Andy
OK. I think…
Basically, I’m trying to understand how two subscripts relate to one another. When I find sign and zero extensions, life gets confusing. In an effort to keep life simple, I begin by walking though the expressions, trying to eliminate extensions where it won’t change the answer. For example, I think
(sext i32 {2,+,1}<%for.body> to i64)
is the same as
{2,+,1}<%for.body>
right?
Mechanically, when I see an sext over an addrec and the addrec has the NW flag, then I can rewrite is as an addrec with the base and step extended. In this case, the base and step are constants, which are particularly easy.
On the other hand, if the addrec is missing the NW flag, I’d be making a mistake.
In a similar vein, it seems plausible that I can rewrite a sign-extend over an add (or multiply), as long as the add (multiply) has the NSW flag, right? Same for zero-extend, over add with NUW flag.
Thanks,
Preston
Sorry, I probably led you astray. No-self-wrap is useful for determining trip count, but does not mean that sign/zero extension can be hoisted.
But if you run your analysis after -indvars, the sign-extension should be removed if possible. The algorithm walks the derived induction variables specifically looking for add nsw/nuw and replacing only those IR users with a wide induction variable. See GetExtendedOperandRecurrence.
If you have a situation where the sign extends cannot removed, then you may need your own IR-level analysis to determine whether you can ignore them. Otherwise, you may run into trouble operating on SCEVs that contain sext/zext/truncs. Are all your array indices uniformly sign-extended? I don’t know if this is a good idea, but why can’t you consider the sext operand the array index rather than the gep operand? If you prove that the narrow indices are disjoint, then the extended indices must be disjoint. SCEV operations should work fine on the narrow indices which shouldn’t have any impure type casting operations.
This can all be avoided by limiting your optimization to code that uses pointer-size loop counters!
-Andy
The array indices are out of my control; depends on the code people write. If they’d use long ints for everything, life would be good; but ints happen.
I’ll play with -indvars.
Thanks,
Preston
Ah, -indvars works a treat.
Hi,
Sorry, I probably led you astray. No-self-wrap is useful for determining
trip count, but does not mean that sign/zero extension can be hoisted.But if you run your analysis after -indvars, the sign-extension should be
removed if possible. The algorithm walks the derived induction variables
specifically looking for add nsw/nuw and replacing only those IR users with
a wide induction variable. See GetExtendedOperandRecurrence.If you have a situation where the sign extends cannot removed, then you may
need your own IR-level analysis to determine whether you can ignore them.
Otherwise, you may run into trouble operating on SCEVs that contain
sext/zext/truncs. Are all your array indices uniformly sign-extended? I
don't know if this is a good idea, but why can't you consider the sext
operand the array index rather than the gep operand? If you prove that the
narrow indices are disjoint, then the extended indices must be disjoint.
SCEV operations should work fine on the narrow indices which shouldn't have
any impure type casting operations.This can all be avoided by limiting your optimization to code that uses
pointer-size loop counters!-Andy
The array indices are out of my control; depends on the code people write. If
they'd use long ints for everything, life would be good; but ints happen.
I thought array indices were promoted to a larger size when possible... This
came up with Ada where loops with i8 counters are quite common. Dan added
logic to boost the size of the loop counters, but perhaps it doesn't promote
them beyond i32, or doesn't apply here for some reason?
Ciao, Duncan.
Hi,
Sorry, I probably led you astray. No-self-wrap is useful for determining
trip count, but does not mean that sign/zero extension can be hoisted.But if you run your analysis after -indvars, the sign-extension should be
removed if possible. The algorithm walks the derived induction variables
specifically looking for add nsw/nuw and replacing only those IR users with
a wide induction variable. See GetExtendedOperandRecurrence.If you have a situation where the sign extends cannot removed, then you may
need your own IR-level analysis to determine whether you can ignore them.
Otherwise, you may run into trouble operating on SCEVs that contain
sext/zext/truncs. Are all your array indices uniformly sign-extended? I
don't know if this is a good idea, but why can't you consider the sext
operand the array index rather than the gep operand? If you prove that the
narrow indices are disjoint, then the extended indices must be disjoint.
SCEV operations should work fine on the narrow indices which shouldn't have
any impure type casting operations.This can all be avoided by limiting your optimization to code that uses
pointer-size loop counters!-Andy
The array indices are out of my control; depends on the code people write. If
they'd use long ints for everything, life would be good; but ints happen.
Random aside: I realize that as a practical engineer, there is value in being
able to just accept certain obstacles as facts of life. However, valuable
engineering insights sometimes depend on recognizing the source of obstacles,
rather than just the ability to cleverly work around them, so there is value
in keeping in mind the fact that this whole situation is caused by a quirk of C
in common usage.
I thought array indices were promoted to a larger size when possible... This
came up with Ada where loops with i8 counters are quite common. Dan added
logic to boost the size of the loop counters, but perhaps it doesn't promote
them beyond i32, or doesn't apply here for some reason?
The pass that does this is -indvars, and it does promote beyond i32 as appropriate
in many common cases, and it sounds like it's working here, from subsequent
messages.
Ciao, Duncan.
I'll play with -indvars.
Dan