fstat + fmtIntSizeBin causes huge substantial .rodata section

[motiejus]

Zig Version

0.10.0-dev.2880+6f0807f50

Steps to Reproduce

Compile this:

test.zig

const std = @import("std");
const fmtIntSizeBin = std.fmt.fmtIntSizeBin;

pub fn main() void {
    const stat = std.os.fstat(0) catch return;
    const file_size = stat.size;
    std.debug.print("{s}\n", .{fmtIntSizeBin(@intCast(u64, file_size))});
}

Build and analyze:

$ zig build-exe --strip -O ReleaseSmall test.zig && bloaty test 
    FILE SIZE        VM SIZE    
 --------------  -------------- 
  70.6%  31.4Ki  59.8%  31.4Ki    .rodata
   0.0%       0  23.4%  12.3Ki    .bss
  19.1%  8.51Ki  16.2%  8.51Ki    .text
   8.3%  3.70Ki   0.0%       0    [Unmapped]
   0.8%     384   0.0%       0    [ELF Section Headers]
   0.6%     280   0.5%     280    [ELF Program Headers]
   0.2%     103   0.0%       0    .comment
   0.1%      64   0.1%      64    [ELF Header]
   0.1%      41   0.0%       0    .shstrtab
   0.0%       8   0.0%       8    [LOAD #1 [R]]
 100.0%  44.5Ki 100.0%  52.6Ki    TOTAL

Notice that .rodata section is over 30KiB, as it seems to include the full errol3 table, as well as some other stuff I wasn't able to recognize.

Removing either fstat (by changing to a constant) or fmtIntSizeBin (by changing to a raw value) reduces the .rodata section to a few hundred bytes. Desired outcome: the .rodata section of the example above should also be a few hundred bytes. I am not sure why a combination of those causes it to include that much stuff, but, well, it does.

The result is the same with both stage1 and stage2.

[Vexu]

I am not sure why a combination of those causes it to include the miscellaneous stuff, but, well, it does.

My guess is that LLVM cannot aggressively inline the float formatting when the value is not known at compile time.

Desired outcome: the .rodata section of the example above should also be a few hundred bytes.

I don't think that is possible with the current float formatting implementation.

Relevant #1299

[motiejus]

Thanks, spot on. Will optimize the floats.