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Replace TimeDateStamp with deterministic hash for reproducible builds #21191

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Replace TimeDateStamp with deterministic hash for reproducible builds #21191

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c01b830
Replace TimeDateStamp with deterministic hash
abulgit Apr 9, 2025
ba2f380
using root.hash
abulgit Apr 10, 2025
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29 changes: 25 additions & 4 deletions compiler/src/dmd/backend/mscoffobj.d
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Original file line number Diff line number Diff line change
Expand Up @@ -35,6 +35,7 @@ import dmd.backend.type;
import dmd.backend.mscoff;

import dmd.common.outbuffer;
import dmd.root.hash;

nothrow:
@safe:
Expand Down Expand Up @@ -649,8 +650,28 @@ void MsCoffObj_term(const(char)[] objfilename)
BIGOBJ_HEADER header = void;
IMAGE_FILE_HEADER header_old = void;

time_t f_timedat = 0;
time(&f_timedat);
// Use a hash of the object content instead of current timestamp for reproducible builds
uint calcDeterministicTimestamp()
{
uint hash = 0;

// Hash each segment
for (segidx_t seg = 1; seg < SegData.length; seg++)
{
seg_data* pseg = SegData[seg];
if (pseg.SDbuf && pseg.SDbuf.length())
{
uint segHash = calcHash(cast(const(ubyte)[])pseg.SDbuf.buf[0..pseg.SDbuf.length()]);

// Mix this segment's hash into the overall hash
hash = cast(uint)mixHash(hash, segHash);
}
}

return hash;
}

uint deterministicTimestamp = calcDeterministicTimestamp();
uint symtable_offset;

if (bigobj)
Expand All @@ -660,7 +681,7 @@ void MsCoffObj_term(const(char)[] objfilename)
header.Version = 2;
header.Machine = I64 ? IMAGE_FILE_MACHINE_AMD64 : IMAGE_FILE_MACHINE_I386;
header.NumberOfSections = scnhdr_cnt;
header.TimeDateStamp = cast(uint)f_timedat;
header.TimeDateStamp = deterministicTimestamp;
static immutable ubyte[16] uuid =
[ '\xc7', '\xa1', '\xba', '\xd1', '\xee', '\xba', '\xa9', '\x4b',
'\xaf', '\x20', '\xfa', '\xf6', '\x6a', '\xa4', '\xdc', '\xb8' ];
Expand All @@ -677,7 +698,7 @@ void MsCoffObj_term(const(char)[] objfilename)
{
header_old.Machine = I64 ? IMAGE_FILE_MACHINE_AMD64 : IMAGE_FILE_MACHINE_I386;
header_old.NumberOfSections = cast(ushort)scnhdr_cnt;
header_old.TimeDateStamp = cast(uint)f_timedat;
header_old.TimeDateStamp = deterministicTimestamp;
header_old.SizeOfOptionalHeader = 0;
header_old.Characteristics = 0;
foffset = (header_old).sizeof; // start after header
Expand Down
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