[llvm-dev] alloca combining, not (yet) possible ?

[Björn Steinbrink via llvm-dev]

HI Nat,

LLVM currently only performs stack coloring to merge allocas if you
use lifetime intrinsics to tell it exactly where the lifetimes of the
alloca start and end. With your code, the lifetimes of both x and y
cover the entire function. Introducing a lexical scope to limit the
lifetime of x gives clang the necessary information to emit the
lifetime.end intrinsic, and declaring y after that scope makes it emit
the lifetime.start intrinsic appropriately as well.

struct a_b {
  long a;
  long b;
};

void f(struct a_b*);

void g(void)
{
  { // Lifetime of x starts here
    struct a_b   x;

    x.a = 1;
    x.b = 2;
    f(&x);
  } // Lifetime of x ends here

  // Lifetime of y starts here
  struct a_b   y;
  y.a = 1;
  y.b = 3;
  f(&y);
  // Lifetime of y ends here
}

It would be nice if LLVM could do this for non-escaping allocas
without the need for those intrinsics, but currently, this is the way
to go.

Cheers,
Björn


2015-08-31 15:21 GMT+02:00 Nat! via llvm-dev <llvm-dev at lists.llvm.org>:
> Caldarale, Charles R schrieb:
>>
>> You have not provided us with the declaration for f().  Unless its
>> argument is marked with the nocapture attribute, the compilation of g()
>> cannot assume that f() has not retained a pointer to the x struct and is
>> using it in the second call.
>>
>
> thanks a lot for the input. Yes, I forgot to that. The C function
> declaration would have been
>
>         void    f( struct a_b *p);
>
> which compiled into
>
>         declare void @f(%struct.a_b*) #2
>
> with
>
>         attributes #2 = { "disable-tail-calls"="false"
> "less-precise-fpmad"="false" "no-frame-pointer-elim"="true"
> "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false"
> "no-nans-fp-math"="false" "stack-protector-buffer-size"="8"
> "target-cpu"="core2" "target-features"="+cx16,+sse,+sse2,+sse3,+ssse3"
> "unsafe-fp-math"="false" "use-soft-float"="false" }
>
> ---
>
> I could not figure out how to decorate my C code to emit the nocapture
> attribute, __attribute(( nocapture) is unknown. So I tried to modify the IR
> code by hand to read thusly:
>
>         declare void @f(%struct.a_b* nocapture) #1
>
> But in the end, it didn't make a difference, when I compiled it with
>
> ../llvm-build.d/bin/llc -O3 -o test-combine-alloca.s test-combine-alloca.ir
>
> it still used two allocas.
>
> From a C perspective, I find it weird, that it should concern the caller if
> the called function "mistakenly" holds onto an alloca buffer, that will be
> invalid soon anyway. But I guess that's C++ magic somehow :)
>
> Ciao
>    Nat!
> ----
> ; ModuleID = 'test-combine-alloca.c'
> target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
> target triple = "x86_64-apple-macosx10.10.0"
>
> %struct.a_b = type { i32, i32 }
>
> declare void @f(%struct.a_b* nocapture) #1
>
> ; Function Attrs: nounwind ssp uwtable
> define void @g() #0 {
> entry:
>   %x = alloca %struct.a_b, align 4
>   %y = alloca %struct.a_b, align 4
>   %a = getelementptr inbounds %struct.a_b, %struct.a_b* %x, i32 0, i32 0
>   store i32 1, i32* %a, align 4
>   %b = getelementptr inbounds %struct.a_b, %struct.a_b* %x, i32 0, i32 1
>   store i32 2, i32* %b, align 4
>   call void @f(%struct.a_b* %x)
>   %a1 = getelementptr inbounds %struct.a_b, %struct.a_b* %y, i32 0, i32 0
>   store i32 1, i32* %a1, align 4
>   %b2 = getelementptr inbounds %struct.a_b, %struct.a_b* %y, i32 0, i32 1
>   store i32 3, i32* %b2, align 4
>   call void @f(%struct.a_b* %y)
>   ret void
> }
>
>
> attributes #0 = { nounwind ssp uwtable "disable-tail-calls"="false"
> "less-precise-fpmad"="false" "no-frame-pointer-elim"="true"
> "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false"
> "no-nans-fp-math"="false" "stack-protector-buffer-size"="8"
> "target-cpu"="core2" "target-features"="+cx16,+sse,+sse2,+sse3,+ssse3"
> "unsafe-fp-math"="false" "use-soft-float"="false" }
> attributes #1 = { "disable-tail-calls"="false" "less-precise-fpmad"="false"
> "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf"
> "no-infs-fp-math"="false" "no-nans-fp-math"="false"
> "stack-protector-buffer-size"="8" "target-cpu"="core2"
> "target-features"="+cx16,+sse,+sse2,+sse3,+ssse3" "unsafe-fp-math"="false"
> "use-soft-float"="false" }
>
> !llvm.module.flags = !{!0}
> !llvm.ident = !{!1}
>
> !0 = !{i32 1, !"PIC Level", i32 2}
> !1 = !{!"clang version 3.7.0 (http://llvm.org/git/clang.git
> 36ba449caa88f710520cdce148457e5a75e9dabc) (http://llvm.org/git/llvm.git
> dccade93466c50834dbaa5f4dabb81e90d768c40)"}
> ----
>
>
>
>
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