[libc][arm] syscalls doesn't build under -mthumb
#93738
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@llvm/issue-subscribers-libc Author: Nick Desaulniers (paternity leave) (nickdesaulniers)
```
In file included from external/llvm-libc/test/UnitTest/TestLogger.cpp:4:
In file included from external/llvm-libc/src/__support/OSUtil/io.h:19:
In file included from external/llvm-libc/src/__support/OSUtil/linux/io.h:13:
In file included from external/llvm-libc/src/__support/OSUtil/linux/syscall.h:21:
external/llvm-libc/src/__support/OSUtil/linux/arm/syscall.h:15:2: error: "The arm syscall implementation does not yet support thumb flavor."
15 | #error "The arm syscall implementation does not yet support thumb flavor."
| ^
```
There's a TODO in the sources without a corresponding bug on file.
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nickdesaulniers
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Jun 25, 2024
r7 is reserved in thumb2 (typically for the frame pointer, as opposed to r11 in ARM mode), so assigning to a variable with explicit register storage in r7 will produce an error. But r7 is where the Linux kernel expects the syscall number to be placed. We can use a temporary to get the register allocator to pick a temporary, which we save+restore the previous value of r7 in. Fixes: #93738
AlexisPerry
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Jul 9, 2024
r7 is reserved in thumb2 (typically for the frame pointer, as opposed to r11 in ARM mode), so assigning to a variable with explicit register storage in r7 will produce an error. But r7 is where the Linux kernel expects the syscall number to be placed. We can use a temporary to get the register allocator to pick a temporary, which we save+restore the previous value of r7 in. Fixes: llvm#93738
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There's a TODO in the sources without a corresponding bug on file.
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