[llvm-dev] What does a dead register mean?

Krzysztof Parzyszek via llvm-dev llvm-dev at lists.llvm.org
Tue Feb 6 06:10:31 PST 2018

You are right about your interpretation of "dead". The case here is that 
RSP is a reserved register and so its liveness isn't really tracked. The 
"implicit-def dead" is an idiom used to mean that the register (reserved 
or not) is clobbered. The other implicit uses/defs can come from 
instruction definitions to indicate that this instruction uses and/or 
modifies a given register (regardless of its explicit operands), but for 
reserved registers there doesn't need to be any continuity between the 
defs and the uses.


On 2/5/2018 11:14 PM, Bhatu via llvm-dev wrote:
> Hi,
> My understanding of a "dead" register is a def that is never used. However,
> when I dump the MI after reg alloc on a simple program I see the 
> following sequence:
> ADJCALLSTACKDOWN64 0, 0, 0, *implicit-def dead %rsp*, implicit-def dead 
> %eflags, implicit-def dead %ssp, implicit %rsp, implicit %ssp
> CALL64pcrel32 @foo, <regmask %bh %bl %bp %bpl %bx %ebp %ebx %rbp %rbx 
> %r12 %r13 %r14 %r15 %r12b %r13b %r14b %r15b %r12d %r13d %r14d %r15d 
> %r12w %r13w %r14w %r15w>, *implicit %rsp*, implicit %ssp, implicit-def 
> %rsp, implicit-def %ssp
> ADJCALLSTACKUP64 0, 0, implicit-def dead %rsp, implicit-def dead 
> %eflags, implicit-def dead %ssp, implicit %rsp, implicit %ssp
> RET 0
> The ADJCALLSTACKDOWN64 has implicit-def dead %rsp. However the next 
> instruction,
> CALL64pcrel32 has an implicit use of %rsp. This would be a use of %rsp 
> as defined
> in ADJCALLSTACKDOWN64 making that non-dead.
> So I guess my understanding of dead is incorrect. Could you please 
> explain what dead means?
> For reference:
> Source file(a.c):
> void foo(void);
> void boo(){ foo(); }
> Commands:
> clang -S -emit-llvm -Xclang -disable-O0-optnone a.c
> llc -print-after="stack-slot-coloring" a.ll
> -- 
> Regards
> Bhatu
> _______________________________________________
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> llvm-dev at lists.llvm.org
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