$ cat alloca.c
void func(void *buf);
void func2(int a) {
    void *buf = _alloca(a);
    func(buf);
}
$ cl -c alloca.c -O2 -GS-
Microsoft (R) C/C++ Optimizing Compiler Version 19.12.25830.2 for ARM
Copyright (C) Microsoft Corporation.  All rights reserved.

alloca.c
$ dumpbin -disasm alloca.obj
Microsoft (R) COFF/PE Dumper Version 14.12.25830.2
Copyright (C) Microsoft Corporation.  All rights reserved.


Dump of file alloca.obj

File Type: COFF OBJECT

func2:
  00000000: B598      push        {r3,r4,r7,lr}
  00000002: 466F      mov         r7,sp
  00000004: 1DC3      adds        r3,r0,#7
  00000006: 4283      cmp         r3,r0
  00000008: D801      bhi         $LN3
  0000000A: F06F 0307 mvn         r3,#7
$LN3:
  0000000E: F023 0307 bic         r3,r3,#7
  00000012: 089C      lsrs        r4,r3,#2
  00000014: F000 F800 bl          __chkstk
  00000018: EBAD 0D04 sub         sp,sp,r4
  0000001C: 4668      mov         r0,sp
  0000001E: F000 F800 bl          func
  00000022: 46BD      mov         sp,r7
  00000024: BD98      pop         {r3,r4,r7,pc}


$ cat vla.c
void func(void *buf);
void func2(int a) {
    char *buf[a];
    func(buf);
}
$ clang -target armv7-win32-msvc -S -emit-llvm alloca.c -O2
$ cat alloca.ll
; ModuleID = 'alloca.c'
source_filename = "alloca.c"
target datalayout = "e-m:w-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
target triple = "thumbv7--windows-msvc19.11.0"

; Function Attrs: nounwind
define arm_aapcs_vfpcc void @func2(i32 %a) local_unnamed_addr #0 {
entry:
  %0 = alloca i8, i32 %a, align 8
  call arm_aapcs_vfpcc void @func(i8* nonnull %0) #2
  ret void
}

$ clang -target armv7-win32-msvc -S -emit-llvm vla.c -O2
$ cat vla.ll
; ModuleID = 'vla.c'
source_filename = "vla.c"
target datalayout = "e-m:w-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
target triple = "thumbv7--windows-msvc19.11.0"

; Function Attrs: nounwind
define arm_aapcs_vfpcc void @func2(i32 %a) local_unnamed_addr #0 {
entry:
  %vla = alloca i8*, i32 %a, align 4
  %0 = bitcast i8** %vla to i8*
  call arm_aapcs_vfpcc void @func(i8* nonnull %0) #2
  ret void
}