diff options
Diffstat (limited to 'lib/Target/X86/README-X86-64.txt')
| -rw-r--r-- | lib/Target/X86/README-X86-64.txt | 49 |
1 files changed, 49 insertions, 0 deletions
diff --git a/lib/Target/X86/README-X86-64.txt b/lib/Target/X86/README-X86-64.txt index ad12137..e8f7c5d 100644 --- a/lib/Target/X86/README-X86-64.txt +++ b/lib/Target/X86/README-X86-64.txt @@ -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. + +//===---------------------------------------------------------------------===// |
