Bug 21364 – Improperly aligned struct when one member is a GPR and the other is an XMM

test.d --------------------- struct X { float x0; long x1; } void foo(int bar, X x, int i1, int i2, int i3, int i4, int i5, int i6, int i7, int i8, int i9) { import std.stdio; writeln(bar); // prints 0 instead of 1 } void main() { X x = X(); foo(1, x, 2, 3, 4, 5, 6, 7, 8, 9, 10); } --------------------- $ dmd -ofapp test.d && ./app The writeln prints 0 instead of 1. $ dmd --version DMD64 D Compiler v2.094.0 The call to foo is compiled to: 9c9ec: 48 83 ec 08 sub $0x8,%rsp 9c9f0: 6a 01 pushq $0x1 9c9f2: ff 75 f8 pushq -0x8(%rbp) 9c9f5: ff 75 f0 pushq -0x10(%rbp) 9c9f8: 48 83 ec 08 sub $0x8,%rsp <----- wrong !!! 9c9fc: 6a 02 pushq $0x2 9c9fe: 6a 03 pushq $0x3 9ca00: 6a 04 pushq $0x4 9ca02: 41 b9 05 00 00 00 mov $0x5,%r9d 9ca08: 41 b8 06 00 00 00 mov $0x6,%r8d 9ca0e: b9 07 00 00 00 mov $0x7,%ecx 9ca13: ba 08 00 00 00 mov $0x8,%edx 9ca18: be 09 00 00 00 mov $0x9,%esi 9ca1d: bf 0a 00 00 00 mov $0xa,%edi 9ca22: e8 91 ff ff ff callq 9c9b8 <_D4test3fooFiSQm1XiiiiiiiiiZv> After pushing the contents of the struct value x onto the stack, rsp is additionally decreased by 8. However, when accessing the function argument later on this additional space of 8 bytes is not taken into account. Smells like an unnecessary alignment padding for struct by-value function call arguments. Either the padding itself is wrong, or the offset in the symbol table does not include the padding. Any small deviation from the code above "fixes" the issue, e.g. removing function arguments from the back, or changing the "float" to "long", or adding extern (C).

regression in pull request #10200

(In reply to aneas from comment #1) > regression in pull request #10200 https://github.com/dlang/dmd/pull/10200

Because of alignment, X.sizeof is 16. So the call is correct, but foo() gets it wrong and treats the size as 12.

Correction: both functions get the size of X right at 16. But, the alignment of X should be 8. Function foo() gets it right, while main() sees the alignment as 16.

@WalterBright created dlang/dmd pull request #11941 "fix Issue 21364 - Improperly aligned struct when one member is a GPR …" fixing this issue: - fix Issue 21364 - Improperly aligned struct when one member is a GPR and the other is an XMM https://github.com/dlang/dmd/pull/11941

dlang/dmd pull request #11941 "fix Issue 21364 - Improperly aligned struct when one member is a GPR …" was merged into master: - 34bd6a00411e6119629f2cd273f46bdd7182e894 by Walter Bright: fix Issue 21364 - Improperly aligned struct when one member is a GPR and the other is an XMM https://github.com/dlang/dmd/pull/11941