From d644235a04be422da53db0d09a74127ae17e462e Mon Sep 17 00:00:00 2001
From: Tom Dohrmann <erbse.13@gmx.de>
Date: Fri, 9 Feb 2024 07:38:14 +0100
Subject: [PATCH] optimize from_page_table_indices

The new code behaves exactly the same as the old code, but can be much
better optimized by LLVM. In particular the bounds checks previously
implied through `set_bits` and `VirtAddr::new` can now be elided. This
also increases the chances of these functions being inlined.

The generated assembly for the old implementation looks like this:
```asm
x86_64::structures::paging::page::Page::from_page_table_indices:
 push    rax
 movzx   edi, di
 cmp     edi, 512
 jae     .LBB48_8
 movzx   esi, si
 cmp     esi, 512
 jae     .LBB48_8
 movzx   edx, dx
 cmp     edx, 512
 jae     .LBB48_8
 movzx   eax, cx
 cmp     eax, 512
 jae     .LBB48_8
 mov     rcx, rdi
 shl     rcx, 39
 shl     rsi, 30
 or      rsi, rcx
 shl     rdx, 21
 shl     rax, 12
 or      rax, rdx
 or      rax, rsi
 shr     edi, 8
 je      .LBB48_7
 cmp     edi, 1
 jne     .LBB48_9
 shl     rax, 16
 sar     rax, 16
.LBB48_7:
 shl     rax, 16
 sar     rax, 16
 and     rax, -4096
 pop     rcx
 ret
.LBB48_8:
 lea     rdi, [rip, +, .L__unnamed_46]
 lea     rdx, [rip, +, .L__unnamed_15]
 mov     esi, 33
 call    qword, ptr, [rip, +, _ZN4core9panicking5panic17h87fd92496103e3b8E@GOTPCREL]
.LBB48_9:
 mov     qword, ptr, [rsp], rax
 lea     rdi, [rip, +, .L__unnamed_9]
 lea     rcx, [rip, +, .L__unnamed_10]
 lea     r8, [rip, +, .L__unnamed_11]
 mov     rdx, rsp
 mov     esi, 74
 call    qword, ptr, [rip, +, _ZN4core6result13unwrap_failed17hc28f4ee1a6255957E@GOTPCREL]
```

The generated assembly for the new implementation looks like this:
```asm
x86_64::structures::paging::page::Page::from_page_table_indices:
 shl     rdi, 55
 movzx   esi, si
 shl     rsi, 46
 or      rsi, rdi
 movzx   edx, dx
 shl     rdx, 37
 movzx   eax, cx
 shl     rax, 28
 or      rax, rdx
 or      rax, rsi
 sar     rax, 16
 ret
```
---
 src/structures/paging/page.rs | 30 ++++++++++++------------------
 1 file changed, 12 insertions(+), 18 deletions(-)

diff --git a/src/structures/paging/page.rs b/src/structures/paging/page.rs
index 88494aef..10414139 100644
--- a/src/structures/paging/page.rs
+++ b/src/structures/paging/page.rs
@@ -182,12 +182,10 @@ impl Page<Size1GiB> {
         p4_index: PageTableIndex,
         p3_index: PageTableIndex,
     ) -> Self {
-        use bit_field::BitField;
-
         let mut addr = 0;
-        addr.set_bits(39..48, u64::from(p4_index));
-        addr.set_bits(30..39, u64::from(p3_index));
-        Page::containing_address(VirtAddr::new(addr))
+        addr |= u64::from(p4_index) << 39;
+        addr |= u64::from(p3_index) << 30;
+        Page::containing_address(VirtAddr::new_truncate(addr))
     }
 }
 
@@ -199,13 +197,11 @@ impl Page<Size2MiB> {
         p3_index: PageTableIndex,
         p2_index: PageTableIndex,
     ) -> Self {
-        use bit_field::BitField;
-
         let mut addr = 0;
-        addr.set_bits(39..48, u64::from(p4_index));
-        addr.set_bits(30..39, u64::from(p3_index));
-        addr.set_bits(21..30, u64::from(p2_index));
-        Page::containing_address(VirtAddr::new(addr))
+        addr |= u64::from(p4_index) << 39;
+        addr |= u64::from(p3_index) << 30;
+        addr |= u64::from(p2_index) << 21;
+        Page::containing_address(VirtAddr::new_truncate(addr))
     }
 }
 
@@ -218,14 +214,12 @@ impl Page<Size4KiB> {
         p2_index: PageTableIndex,
         p1_index: PageTableIndex,
     ) -> Self {
-        use bit_field::BitField;
-
         let mut addr = 0;
-        addr.set_bits(39..48, u64::from(p4_index));
-        addr.set_bits(30..39, u64::from(p3_index));
-        addr.set_bits(21..30, u64::from(p2_index));
-        addr.set_bits(12..21, u64::from(p1_index));
-        Page::containing_address(VirtAddr::new(addr))
+        addr |= u64::from(p4_index) << 39;
+        addr |= u64::from(p3_index) << 30;
+        addr |= u64::from(p2_index) << 21;
+        addr |= u64::from(p1_index) << 12;
+        Page::containing_address(VirtAddr::new_truncate(addr))
     }
 
     /// Returns the level 1 page table index of this page.