operator new[] overflow (PR 19351)

[Chris Lattner]

On Dec 5, 2010, at 9:49 AM, Chris Lattner wrote:

> 
> On Dec 5, 2010, at 3:19 AM, Richard Guenther wrote:
> 
>>> $ clang t.cc -S -o - -O3 -mkernel -fomit-frame-pointer -mllvm -show-mc-encoding
>>>       .section        __TEXT,__text,regular,pure_instructions
>>>       .globl  __Z4testl
>>>       .align  4, 0x90
>>> __Z4testl:                              ## @_Z4testl
>>> ## BB#0:                                ## %entry
>>>       movl    $4, %ecx                ## encoding: [0xb9,0x04,0x00,0x00,0x00]
>>>       movq    %rdi, %rax              ## encoding: [0x48,0x89,0xf8]
>>>       mulq    %rcx                    ## encoding: [0x48,0xf7,0xe1]
>>>       movq    $-1, %rdi               ## encoding: [0x48,0xc7,0xc7,0xff,0xff,0xff,0xff]
>>>       cmovnoq %rax, %rdi              ## encoding: [0x48,0x0f,0x41,0xf8]
>>>       jmp     __Znam                  ## TAILCALL
>>>                                       ## encoding: [0xeb,A]
>>>                                       ##   fixup A - offset: 1, value: __Znam-1, kind: FK_PCRel_1
>>> .subsections_via_symbols
>>> 
>>> This could be further improved by inverting the cmov condition to avoid the first movq, which we'll tackle as a general regalloc improvement.
>> 
>> I'm curious as on how you represent the overflow checking in your highlevel IL.
> 
> The (optimized) generated IR is:
> 
> $ clang t.cc -emit-llvm -S -o - -O3
> ...
> define noalias i8* @_Z4testl(i64 %count) ssp {
> entry:
>  %0 = tail call %0 @llvm.umul.with.overflow.i64(i64 %count, i64 4)
>  %1 = extractvalue %0 %0, 1
>  %2 = extractvalue %0 %0, 0
>  %3 = select i1 %1, i64 -1, i64 %2
>  %call = tail call noalias i8* @_Znam(i64 %3)
>  ret i8* %call
> }

Sorry, it's a little easier to read with expanded names and types:

define noalias i8* @_Z4testl(i64 %count) ssp {
entry:
  %A = tail call { i64, i1 } @llvm.umul.with.overflow.i64(i64 %count, i64 4)
  %B = extractvalue { i64, i1 } %A, 1
  %C = extractvalue { i64, i1 } %A, 0
  %D = select i1 %B, i64 -1, i64 %C
  %call = tail call noalias i8* @_Znam(i64 %D)
  ret i8* %call
}

-Chris