http://llvm.org/bugs/show_bug.cgi?id=4242
Summary: [x86-64 ABI] struct passed partially in registers if
there's an sret parameter
Product: new-bugs
Version: unspecified
Platform: PC
OS/Version: Linux
Status: NEW
Keywords: ABI
Severity: normal
Priority: P2
Component: new bugs
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected]
Created an attachment (id=3013)
--> (http://llvm.org/bugs/attachment.cgi?id=3013)
The code in question
There's an ABI bug in llvm-gcc for x86-64 linux.
The attached code should generate functionally identical asm for both
functions, and does so with stock g++ (version: Ubuntu 4.3.2-1ubuntu12) but not
with llvm-g++; it generates incorrect code for fail().
The issue seems to be that the sret pointer isn't taken into account when the
number of available integer registers for the struct parameter is calculated
causing that parameter to be passed with one half in a register and one in
memory (in violation of the ABI spec, which requires it to be passed entirely
in memory if it does not fit entirely into registers).
g++ output (via objdump)
----
0000000000000000 <fail>:
0: 48 8b 44 24 08 mov 0x8(%rsp),%rax
5: 4c 89 4f 10 mov %r9,0x10(%rdi)
9: 48 89 07 mov %rax,(%rdi)
c: 48 8b 44 24 10 mov 0x10(%rsp),%rax
11: 48 89 47 08 mov %rax,0x8(%rdi)
15: 48 89 f8 mov %rdi,%rax
18: c3 retq
----
llvm-g++ output:
----
0000000000000000 <fail>:
0: 48 89 f8 mov %rdi,%rax
3: 4c 89 0f mov %r9,(%rdi)
6: 48 8b 4c 24 08 mov 0x8(%rsp),%rcx
b: 48 89 4f 08 mov %rcx,0x8(%rdi)
f: 48 8b 4c 24 10 mov 0x10(%rsp),%rcx
14: 48 89 4f 10 mov %rcx,0x10(%rdi)
18: c3 retq
----
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