| Issue |
169588
|
| Summary |
Win ARM64 prolog prevents use of packed unwind info
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
LouisLaf
|
The prolog for Win ARM64 functions allocates stacks for the parameter home area and locals with a single SUB. While this sometimes results in 1 less instruction in the prolog, it does prevent taking advantage of the packed unwind data format.
For example, for this test case:
extern void bar(int*);
```
int foo(int x)
{
int z;
bar(&z);
return x+z;
}
```
LLVM emits this prolog:
```
sub sp,sp,#0x20
str x19,[sp,#0x10]
str lr,[sp,#0x18]
```
This results in an unpacked unwind info record.
Emitting this prolog instead:
```
sub sp,sp,#0x10
stp x19,lr,[sp]
sub sp,sp,#0x10
```
Would allow to take advantage of the packed unwind data format according to [https://learn.microsoft.com/en-us/cpp/build/arm64-exception-handling](url)
Using packed records reduce the size of the unwind information in rdata significantly and improves the speed of stack walking.
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