[llvm-dev] Optimization generate super long function definition

Jason -Zhong Sheng- Hu via llvm-dev llvm-dev at lists.llvm.org
Sun Apr 12 15:49:37 PDT 2020

Hi all,

sorry to have sent the same question around. I am quite desperately looking for a solution to this problem and I figured the mailing list is the best bet.

In my code, I generate the following function:

define i32 @gl.qi([500 x i32] %x, i32 %i) {
  %x. = alloca [500 x i32]
  %i. = alloca i32
  %0 = alloca [500 x i32]
  store [500 x i32] %x, [500 x i32]* %x.
  store i32 %i, i32* %i.
  %x.1 = load [500 x i32], [500 x i32]* %x.
  %i.2 = load i32, i32* %i.
  store [500 x i32] %x.1, [500 x i32]* %0
  %1 = icmp slt i32 %i.2, 500
  br i1 %1, label %in-bound, label %out-of-bound

out-of-bound:                                     ; preds = %entry
  call void @gen.panic(i8* getelementptr inbounds ([22 x i8], [22 x i8]* @pool.str.2, i32 0, i32 0))

in-bound:                                         ; preds = %entry
  %2 = getelementptr inbounds [500 x i32], [500 x i32]* %0, i32 0, i32 %i.2
  %idx = load i32, i32* %2
  ret i32 %idx

the high level functionality is to use %i to index %x, and if %i is out of bound, a panic function is called instead.

consider the store line:

  store [500 x i32] %x, [500 x i32]* %x.

once I pass this function to opt -O1 -S --verify --verify-each, it generates code like this:

define i32 @gl.qi([500 x i32] %x, i32 %i) local_unnamed_addr {
  %0 = alloca [500 x i32], align 4
  %x.fca.0.extract = extractvalue [500 x i32] %x, 0
  %x.fca.1.extract = extractvalue [500 x i32] %x, 1
  %x.fca.2.extract = extractvalue [500 x i32] %x, 2
  %x.fca.3.extract = extractvalue [500 x i32] %x, 3
  %x.fca.4.extract = extractvalue [500 x i32] %x, 4
  %x.fca.5.extract = extractvalue [500 x i32] %x, 5

until 500. I put the number to 50000 and it won’t stop.

This is puzzling. I am not sure why must a store command be expanded to a sequence of etractvalues then stores? Is there a way to turn off this particular optimization without turning off the whole optimization?

Or am I looking at the wrong way to do this simple task?

Jason Hu
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