[X86] Emit fewer instructions to allocate >16GB stack frames
Summary:
Use this code pattern when RAX is live, instead of emitting up to 2
billion adjustments:
pushq %rax
movabsq +-$Offset+-8, %rax
addq %rsp, %rax
xchg %rax, (%rsp)
movq (%rsp), %rsp
Try to clean this code up a bit while I'm here. In particular, hoist the
logic that handles the entire adjustment with `movabsq $imm, %rax` out
of the loop.
This negates the offset in the prologue and uses ADD because X86 only
has a two operand subtract which always subtracts from the destination
register, which can no longer be RSP.
Fixes PR31962
Reviewers: majnemer, sdardis
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D30052
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298116 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed