[LLVMbugs] [Bug 23809] New: InstCombine should keep useful @llvm.assume calls

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23809

            Bug ID: 23809
           Summary: InstCombine should keep useful @llvm.assume calls
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: wujingyue at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 14461
  --> https://llvm.org/bugs/attachment.cgi?id=14461&action=edit
the test case as mentioned in the description

I have some WIP that leverages @llvm.assume in some optimization passes other
than InstCombine. However, it doesn't work yet because InstCombine removes
@llvm.assume calls that are useful for later optimizations. For example, given

define i32 @foo(i32 %a, i32 %b) {
  %sum = add i32 %a, %b
  %1 = icmp sge i32 %sum, 0
  call void @llvm.assume(i1 %1)
  ret i32 %sum
}

"opt -instcombine" emits

define i32 @foo(i32 %a, i32 %b) {
  %sum = add i32 %a, %b
  ret i32 %sum
}

removing the @llvm.assume call so later optimizations won't know sum is
non-negative any more. (Note that the opt I use here is with
http://reviews.llvm.org/D10283 patched. This patch fixes a bug in
ValueTracking). 

The reasons that InstCombine removes this assume call are: 
1) SimplifyICmpInst proves %1 is true based on the assume call. 
2) then, InstCombine
(http://llvm.org/docs/doxygen/html/InstCombineCompares_8cpp_source.html#l02649)
replaces all uses of %1 with true including the use in the assume call. 
3) Somewhere later, llvm.assume(true) is considered trivially dead and thus
removed from the IR.

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