[llvm] [X86] Truncate i64 sub to i32 when upper 33 bits are zeros (PR #145850)

[Simon Pilgrim via llvm-commits]

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@@ -0,0 +1,78 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=i686-unknown-unknown | FileCheck %s --check-prefix=X86
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown | FileCheck %s --check-prefix=X64
+
+; Truncate to 32 bit subtraction since first 48 bits are zeros
+define i64 @test1(i16 %a, i16 %b) nounwind {
+; X86-LABEL: test1:
+; X86:       # %bb.0:
+; X86-NEXT:    movzwl {{[0-9]+}}(%esp), %ecx
+; X86-NEXT:    movzwl {{[0-9]+}}(%esp), %eax
+; X86-NEXT:    xorl %edx, %edx
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RKSimon wrote:

Its necessary to stop a dependency on the old value of EDX stalling the SBB instruction - some cpus recognise the dependency break (search X86ScheduleBtVer2.td for IsDepBreakingFunction). 

https://github.com/llvm/llvm-project/pull/145850