[PATCH] x86 NaCl: Emit LAHF/SAHF instead of PUSHF/POPF

[JF Bastien]

At this point I think the issue is in how liveness is tagged onto `CMOV`, but I'm not sure I understand how that happens from the tablegen files.  Post-RA pseudo instruction expansion pass transforms the following:

  	CALL64pcrel32 <ga:@foo>, <regmask>, %RSP<imp-use>, %RSP<imp-def>, %EAX<imp-def>
  	%EFLAGS<def> = COPY %R14<kill>
  	%EAX<def,tied1> = CMOVE32rr %EAX<kill,tied0>, %EBX<kill>, %EFLAGS<imp-use>
  	%RSP<def,tied1> = ADD64ri8 %RSP<tied0>, 8, %EFLAGS<imp-def,dead>
  	%RBX<def> = POP64r %RSP<imp-def>, %RSP<imp-use>
  	%R14<def> = POP64r %RSP<imp-def>, %RSP<imp-use>
  	RETQ %EAX

Into:

  	CALL64pcrel32 <ga:@foo>, <regmask>, %RSP<imp-use>, %RSP<imp-def>, %EAX<imp-def>
  	%RAX<def> = MOV64rr %R14<kill>
  	%AL<def,tied1> = ADD8ri %AL<tied0>, 127, %EFLAGS<imp-def>
  	SAHF %EFLAGS<imp-def>, %AH<imp-use>
  	%EAX<def,tied1> = CMOVE32rr %EAX<kill,tied0>, %EBX<kill>, %EFLAGS<imp-use>
  	%RSP<def,tied1> = ADD64ri8 %RSP<tied0>, 8, %EFLAGS<imp-def,dead>
  	%RBX<def> = POP64r %RSP<imp-def>, %RSP<imp-use>
  	%R14<def> = POP64r %RSP<imp-def>, %RSP<imp-use>
  	RETQ %EAX

Note the lack of AX save/restore because liveness thinks it ins't live at this location.

@t.p.northover do you think that's indeed where the bug lies, or am I missing something?


http://reviews.llvm.org/D6629

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