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2a77d92e2e
This patch addresses the issue regarding the call of bcopy function in a
conditional expression.
It is analogous to the already accepted patch which deals with the same
problem, just regarding the bzero function [0].
Here is the testcase which illustrates the issue:
```
void bcopy(const void *, void *, unsigned long);
void foo(void);
void test_bcopy() {
char dst[20];
char src[20];
int _sz = 20, len = 20;
return (_sz
? ((_sz >= len)
? bcopy(src, dst, len)
: foo())
: bcopy(src, dst, len));
}
```
When processing it with clang, following issue occurs:
Instruction does not dominate all uses!
%arraydecay2 = getelementptr inbounds [20 x i8], ptr %dst, i64 0, i64 0,
!dbg !38
%cond = phi ptr [ %arraydecay2, %cond.end ], [ %arraydecay5,
%cond.false3 ], !dbg !33
fatal error: error in backend: Broken module found, compilation aborted!
This happens because an incorrect phi node is created. It is created
because bcopy function call is lowered to the call of llvm.memmove
intrinsic and function memmove returns void *. Since llvm.memmove is
called in two places in the same return statement, clang creates a phi
node in the final basic block for the return value and that phi node is
incorrect. However, bcopy function should return void in the first
place, so this phi node is unnecessary. This is what this patch
addresses. An appropriate test is also added and no existing tests fail
when applying this patch.
Also, this crash only happens when LLVM is configured with
-DLLVM_ENABLE_ASSERTIONS=On option.
[0] https://reviews.llvm.org/D39746
IRgen optimization opportunities. //===---------------------------------------------------------------------===// The common pattern of -- short x; // or char, etc (x == 10) -- generates an zext/sext of x which can easily be avoided. //===---------------------------------------------------------------------===// Bitfields accesses can be shifted to simplify masking and sign extension. For example, if the bitfield width is 8 and it is appropriately aligned then is is a lot shorter to just load the char directly. //===---------------------------------------------------------------------===// It may be worth avoiding creation of alloca's for formal arguments for the common situation where the argument is never written to or has its address taken. The idea would be to begin generating code by using the argument directly and if its address is taken or it is stored to then generate the alloca and patch up the existing code. In theory, the same optimization could be a win for block local variables as long as the declaration dominates all statements in the block. NOTE: The main case we care about this for is for -O0 -g compile time performance, and in that scenario we will need to emit the alloca anyway currently to emit proper debug info. So this is blocked by being able to emit debug information which refers to an LLVM temporary, not an alloca. //===---------------------------------------------------------------------===// We should try and avoid generating basic blocks which only contain jumps. At -O0, this penalizes us all the way from IRgen (malloc & instruction overhead), all the way down through code generation and assembly time. On 176.gcc:expr.ll, it looks like over 12% of basic blocks are just direct branches! //===---------------------------------------------------------------------===//