[PATCH] D81765: Don't inline dynamic allocas that simplify to huge static allocas.

[Eli Friedman via Phabricator via llvm-commits]

efriedma added inline comments.


================
Comment at: llvm/lib/Analysis/InlineCost.cpp:838
   if (I.isArrayAllocation()) {
     Constant *Size = SimplifiedValues.lookup(I.getArraySize());
----------------
aemerson wrote:
> efriedma wrote:
> > aemerson wrote:
> > > efriedma wrote:
> > > > Hhow is the handling for array allocation supposed to work if the alloca isn't in the entry block of the function?  It looks like this code assumes that constant-propagation is enough to turn any dynamic alloca into a static alloca?
> > > > 
> > > > Not really a problem with this change, exactly, but I want to make sure I'm understanding the surrounding code correctly.
> > > I'm not entirely sure what you mean, but if an array allocation isn't in the entry block, and if constant prop can't simplify it to a static alloca, then it remains a dynamic alloca and the inliner gives up (see the end of this function).
> > My question is what happens if an array allocation isn't in the entry block, but we can simplify the size to a constant.
> That was the original motivation for this fix actually. If its simplified to a constant, then it can be converted to a static alloca and then later moved into the entry block. If that constant allocation is too large then we can get overflows.
Take the following testcase.  On trunk, we inline into caller1 and caller3, but not caller2 and caller4.  I think your patch stops inlining in caller1, but not caller3.  That seems weird to me...

```
target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.15.0"

define void @caller1(i8 *%p1, i1 %b) {
entry:
  %cond = icmp eq i1 %b, true
  br i1 %cond, label %exit, label %split

split:
  call void @callee(i8* %p1, i32 0, i32 -1)
  br label %exit

exit:
  ret void
}

define void @caller2(i8 *%p1, i1 %b, i32 %len) {
entry:
  %cond = icmp eq i1 %b, true
  br i1 %cond, label %exit, label %split

split:
  call void @callee(i8* %p1, i32 0, i32 %len)
  br label %exit

exit:
  ret void
}

define void @caller3(i8 *%p1, i1 %b, i32 %len) {
entry:
  %cond = icmp eq i1 %b, true
  br i1 %cond, label %exit, label %split

split:
  call void @callee(i8* %p1, i32 0, i32 300)
  br label %exit

exit:
  ret void
}

define void @callee(i8* %p1, i32 %l1, i32 %l2) {
entry:
  %ext = zext i32 %l2 to i64
  br label %foo
foo:
  %vla = alloca float, i64 %ext, align 16
  %call = call i1 @extern_call(float* nonnull %vla) nounwind 
  br i1 %call, label %foo, label %exit
  
exit:
  ret void
}

define void @caller4(i8 *%p1, i1 %b, i32 %len) {
entry:
  %cond = icmp eq i1 %b, true
  br i1 %cond, label %exit, label %split

split:
  call void @callee2(i8* %p1, i32 0)
  br label %exit

exit:
  ret void
}

define void @callee2(i8* %p1, i32 %l1) {
entry:
  br label %foo
foo:
  %vla = alloca float, i64 300, align 16
  %call = call i1 @extern_call(float* nonnull %vla) nounwind 
  br i1 %call, label %foo, label %exit
  
exit:
  ret void
}

declare i1 @extern_call(float*)
```


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D81765/new/

https://reviews.llvm.org/D81765