MaskRay added inline comments.
================
Comment at: compiler-rt/lib/builtins/fp_lib.h:49
-static __inline int rep_clz(rep_t a) { return __builtin_clz(a); }
+static __inline int rep_clz(rep_t a) { return clzsi(a); }
----------------
atrosinenko wrote:
> MaskRay wrote:
> > Why is this needed?
> First of all, I will compile the examples with gcc because right now I most
> probably don't have a TI sysroot for MSP430 compatible with the mainline
> clang and testing with my locally patched llvm+clang seems even less correct
> than with gcc.
>
> The problem is that the definition, according to the [gcc
> documentation](https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html) is
> `int __builtin_clz (unsigned int x)`. This builtin is defined with the
> "default size" `int` argument, not `uint32_t` or `int32_t`. Yes, there are
> `int`s in the specifications that are not exactly `int` but something
> "expected to be int" actually expressed via machine modes. But here it looks
> like it is actually an int:
>
> ```lang=cpp,name=clzsi_test.c
> #include <stdio.h>
> #include <stdint.h>
>
> typedef uint32_t rep_t;
>
> static int rep_clz(rep_t a) { return __builtin_clz(a); }
>
> int main()
> {
> printf("sizeof(int) = %d\n", (int)sizeof(int));
> printf("first: %d\n", rep_clz(0x1000));
> printf("second: %d\n", rep_clz(0x10000));
> printf("third: %d\n", rep_clz(0x100000));
> }
> ```
>
> When compiled and launched on x86_64 Linux host, it prints the expected
> results:
>
> ```
> $ gcc -Wall clzsi_test.c -o clzsi_test && ./clzsi_test
> sizeof(int) = 4
> first: 19
> second: 15
> third: 11
> ```
> Let's compile and run it with
> ```
> $ $sysroot/bin/msp430-elf-gcc --version
> msp430-elf-gcc (Mitto Systems Limited - msp430-gcc 8.3.1.25) 8.3.1
> Copyright (C) 2018 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions. There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> ```
>
> Then we get the following (the `-msim` option is necessary to run the
> compiled binary with `msp430-elf-run` simulator):
>
> ```
> $ $sysroot/bin/msp430-elf-gcc -msim -Wall clzsi_test.c -o clzsi_test &&
> $sysroot/bin/msp430-elf-run ./clzsi_test
> sizeof(int) = 2
> first: 3
> second: 16
> third: 16
> ```
>
> So, these are expected results for `clz(i16 arg)` (with the third line being
> an UB at all) but not the results expected for `rep_t`-sized argument.
> //(When I comment out the third `printf`, other output that now is expected
> to be UB-free remains unchanged)//
>
> When I change `rep_clz` to use `__builtin_clzl`, the output on MSP430
> simulator differs from the host one just in the `sizeof...` line, as expected.
>
> ```
> sizeof(int) = 2
> first: 19
> second: 15
> third: 11
> ```
>
> Of course, now it will output not the expected values on x86_64, so using
> `clzsi` instead of a hardcoded name.
Where is this clzsi defined? It seems that clzsi is provided by either
`__builtin_clz` or `__builtin_clzl`, but the desired version is a customized
one.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D81408/new/
https://reviews.llvm.org/D81408
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