Subzero: Improve handling of alloca instructions of constant size.

tests_lit/llvm2ice_tests/ebp_args.ll

 ; This test originally exhibited a bug in ebp-based stack slots.  The
 ; problem was that during a function call push sequence, the esp
 ; adjustment was incorrectly added to the stack/frame offset for
 ; ebp-based frames.
 
 ; RUN: %p2i --filetype=obj --disassemble -i %s --args -Om1 | FileCheck %s
 
 declare i32 @memcpy_helper2(i32 %buf, i32 %buf2, i32 %n)
 
 define i32 @memcpy_helper(i32 %buf, i32 %n) {
 entry:
+  br label %eblock  ; Disable alloca optimization
+eblock:
   %buf2 = alloca i8, i32 128, align 4
   %n.arg_trunc = trunc i32 %n to i8
   %arg.ext = zext i8 %n.arg_trunc to i32
   %buf2.asint = ptrtoint i8* %buf2 to i32
   %call = call i32 @memcpy_helper2(i32 %buf, i32 %buf2.asint, i32 %arg.ext)
   ret i32 %call
 }
 
 ; This check sequence is highly specific to the current Om1 lowering
 ; and stack slot assignment code, and may need to be relaxed if the
@@ skipping 10 matching lines @@
 ; CHECK:  movzx eax,BYTE PTR [ebp-0x8]
 ; CHECK:  mov   DWORD PTR [ebp-0xc],eax
 ; CHECK:  sub   esp,0x10
 ; CHECK:  mov   eax,DWORD PTR [ebp+0x8]
 ; CHECK:  mov   DWORD PTR [esp],eax
 ; CHECK:  mov   eax,DWORD PTR [ebp-0x4]
 ; CHECK:  mov   DWORD PTR [esp+0x4],eax
 ; CHECK:  mov   eax,DWORD PTR [ebp-0xc]
 ; CHECK:  mov   DWORD PTR [esp+0x8],eax
 ; CHECK:  call {{.*}} R_{{.*}} memcpy_helper2