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add a note about dead zero extends.

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llvm-svn: 78511
This commit is contained in:
Chris Lattner 2009-08-08 22:46:59 +00:00
parent 2849883bd5
commit df9c45048d
1 changed files with 49 additions and 0 deletions
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49
lib/Target/X86/README-X86-64.txt
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@ -249,3 +249,52 @@ lowered return value, and it would free non-C frontends from a
complication only required by a C-based ABI.
//===---------------------------------------------------------------------===//
We get a redundant zero extension for code like this:
int mask[1000];
int foo(unsigned x) {
if (x < 10)
x = x * 45;
else
x = x * 78;
return mask[x];
}
_foo:
LBB1_0: ## entry
cmpl $9, %edi
jbe LBB1_3 ## bb
LBB1_1: ## bb1
imull $78, %edi, %eax
LBB1_2: ## bb2
movl %eax, %eax <----
movq _mask@GOTPCREL(%rip), %rcx
movl (%rcx,%rax,4), %eax
ret
LBB1_3: ## bb
imull $45, %edi, %eax
jmp LBB1_2 ## bb2
Before regalloc, we have:
%reg1025<def> = IMUL32rri8 %reg1024, 45, %EFLAGS<imp-def>
JMP mbb<bb2,0x203afb0>
Successors according to CFG: 0x203afb0 (#3)
bb1: 0x203af60, LLVM BB @0x1e02310, ID#2:
Predecessors according to CFG: 0x203aec0 (#0)
%reg1026<def> = IMUL32rri8 %reg1024, 78, %EFLAGS<imp-def>
Successors according to CFG: 0x203afb0 (#3)
bb2: 0x203afb0, LLVM BB @0x1e02340, ID#3:
Predecessors according to CFG: 0x203af10 (#1) 0x203af60 (#2)
%reg1027<def> = PHI %reg1025, mbb<bb,0x203af10>,
%reg1026, mbb<bb1,0x203af60>
%reg1029<def> = MOVZX64rr32 %reg1027
so we'd have to know that IMUL32rri8 leaves the high word zero extended and to
be able to recognize the zero extend. This could also presumably be implemented
if we have whole-function selectiondags.
//===---------------------------------------------------------------------===//