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Add a note about the inability to model FP -> int conversions which
perform rounding other than truncation in the IR. Common C code for this turns into really an LLVM intrinsic call that blocks a lot of further optimizations. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123135 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -2273,3 +2273,58 @@ this whenever the floating point type has enough exponent bits to represent
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the largest integer value as < inf.
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//===---------------------------------------------------------------------===//
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clang -O3 currently compiles this code:
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#include <emmintrin.h>
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int f(double x) { return _mm_cvtsd_si32(_mm_set_sd(x)); }
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int g(double x) { return _mm_cvttsd_si32(_mm_set_sd(x)); }
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into
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define i32 @_Z1fd(double %x) nounwind readnone {
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entry:
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%vecinit.i = insertelement <2 x double> undef, double %x, i32 0
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%vecinit1.i = insertelement <2 x double> %vecinit.i, double 0.000000e+00, i32 1
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%0 = tail call i32 @llvm.x86.sse2.cvtsd2si(<2 x double> %vecinit1.i) nounwind
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ret i32 %0
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}
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define i32 @_Z1gd(double %x) nounwind readnone {
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entry:
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%conv.i = fptosi double %x to i32
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ret i32 %conv.i
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}
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This difference carries over to the assmebly produced, resulting in:
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_Z1fd: # @_Z1fd
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# BB#0: # %entry
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pushq %rbp
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movq %rsp, %rbp
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xorps %xmm1, %xmm1
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movsd %xmm0, %xmm1
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cvtsd2sil %xmm1, %eax
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popq %rbp
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ret
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_Z1gd: # @_Z1gd
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# BB#0: # %entry
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pushq %rbp
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movq %rsp, %rbp
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cvttsd2si %xmm0, %eax
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popq %rbp
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ret
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The problem is that we can't see through the intrinsic call used for cvtsd2si,
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and fold away the unnecessary manipulation of the function parameter. When
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these functions are inlined, it forms a barrier preventing many further
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optimizations. LLVM IR doesn't have a good way to model the logic of
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'cvtsd2si', its only FP -> int conversion path forces truncation. We should add
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a rounding flag onto fptosi so that it can represent this type of rounding
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naturally in the IR rather than using intrinsics. We might need to use a
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'system_rounding_mode' flag to encode that the semantics of the rounding mode
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can be changed by the program, but ideally we could just say that isn't
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supported, and hard code the rounding.
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//===---------------------------------------------------------------------===//
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