//===---------------------------------------------------------------------===//
-Make use of "Red Zone".
-
-//===---------------------------------------------------------------------===//
-
-Implement __int128 and long double support.
-
-//===---------------------------------------------------------------------===//
-
For this:
extern void xx(void);
instruction. We should probably introduce something like AbsoluteAddress to
distinguish it from GlobalAddress so the asm printer and JIT code emitter can
do the right thing.
+
+//===---------------------------------------------------------------------===//
+
+It's not possible to reference AH, BH, CH, and DH registers in an instruction
+requiring REX prefix. However, divb and mulb both produce results in AH. If isel
+emits a CopyFromReg which gets turned into a movb and that can be allocated a
+r8b - r15b.
+
+To get around this, isel emits a CopyFromReg from AX and then right shift it
+down by 8 and truncate it. It's not pretty but it works. We need some register
+allocation magic to make the hack go away (e.g. putting additional constraints
+on the result of the movb).
+
+//===---------------------------------------------------------------------===//
+
+The x86-64 ABI for hidden-argument struct returns requires that the
+incoming value of %rdi be copied into %rax by the callee upon return.
+
+The idea is that it saves callers from having to remember this value,
+which would often require a callee-saved register. Callees usually
+need to keep this value live for most of their body anyway, so it
+doesn't add a significant burden on them.
+
+We currently implement this in codegen, however this is suboptimal
+because it means that it would be quite awkward to implement the
+optimization for callers.
+
+A better implementation would be to relax the LLVM IR rules for sret
+arguments to allow a function with an sret argument to have a non-void
+return type, and to have the front-end to set up the sret argument value
+as the return value of the function. The front-end could more easily
+emit uses of the returned struct value to be in terms of the function's
+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.
+
+//===---------------------------------------------------------------------===//
projects / oota-llvm.git / blobdiff