https://bugs.llvm.org/show_bug.cgi?id=43257
Bug ID: 43257
Summary: Missed optimization for multiplication on 1.5 and 1.25
Product: clang
Version: unspecified
Hardware: All
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: LLVM Codegen
Assignee: unassignedclangb...@nondot.org
Reporter: zamazan...@tut.by
CC: llvm-bugs@lists.llvm.org, neeil...@live.com,
richard-l...@metafoo.co.uk
On x86_32 for any number X of type (unsigned, unsigned short, unsigned char)
multiplication by 1.5 with a conversion back to unsigned with any rounding mode
produces the exactly same result as if X + (X >> 1).
Same holds for 1.25:
unsigned(X * 1.25) == unsigned(X + (X >> 2))
The above transformation allows to emit a short code without floating point
computations:
test2(unsigned int): # @test2(unsigned int)
mov eax, edi
shr eax
lea eax, [rax + rdi]
ret
Instead of:
test(unsigned int): # @test(unsigned int)
mov eax, edi
cvtsi2sd xmm0, rax
mulsd xmm0, qword ptr [rip + .LCPI0_0]
cvttsd2si rax, xmm0
ret
Same issue in GCC: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91709
Godbolt playground: https://godbolt.org/z/zVQuwP
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