Understanding The Stack

Ewald Cress digs into stack frames, using a few SQL Server functions as examples:

The rdi register is used as a scratchpad to save the incoming rcx value for later – first it is used to restore the possibly clobered rcx value for the second child function call, and then it becomes the return value going into rax. However, since we don’t want to clobber its value for our caller, we first save its old value and then, at the very end, restore it. This is done through a traditional push/pop pair, which in x64 is only allowed in the function prologue and epilogue.

Similarly, ebx (the bottom half of rbx) is used to save the incoming value of the second parameter in edx. However, here we don’t use a push/pop pair, but instead save it in a slot in the shadow space. Why two different mechanisms for two different registers? Sorry, that’s one for the compiler writers to answer.

There’s a lot of detail involved here, but it’s well worth the read.

Related Posts

Understanding Hash Match Aggregates

Itzik Ben-Gan continues his series on grouping and aggregating data by looking at the hash match aggregation process: The estimated CPU cost for the Hash Aggregate in the plan for Query 8 is 0.166344, and in Query 9 is 0.16903. It could be an interesting exercise to try and figure out exactly in what way […]

Read More

Row Width And Snapshot Isolation

Kendra Little shows us the impact that row width has on snapshot isolation: So I went to work to demonstrate row width impact on the version store — when only a tiny bit column is changed in the row. Here’s how I did the test: I created two tables, dbo.Narrow and dbo.Wide. They each each have a […]

Read More

Categories

March 2017
MTWTFSS
« Feb Apr »
 12345
6789101112
13141516171819
20212223242526
2728293031